&EPA
1
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PREVENTION
United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
Office of Pollution
Prevention and Toxics
Washington, DC 20460
EPA/600/R-93/110
August 1993
Pollution Prevention
Technologies for the
Bleached Kraft
Segment of the U.S.
Pulp and Paper Industry
Printed on Recycled Paper
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EPA/600/R-93/110
August 1993
POLLUTION PREVENTION TECHNOLOGIES FOR THE BLEACHED
KRAFT SEGMENT OF THE U.S. PULP AND PAPER INDUSTRY
Contract No. 68-CO-0068
Work Assignment Manager
Jocelyn Woodman
Pollution Prevention Division
Office of Pollution Prevention and Toxics
Office of Pollution Prevention and Toxics
Office of Prevention, Pesticides, and Toxic Substances
U.S. Environmental Protection Agency
Washington, DC 20460
Office of Environmental Engineering and Technology Demonstration
Office of Research and Development
U.S. Environmental Protection Agency
Washington, DC 20460
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Notice
This document is intended to provide technical and economic information on approaches to
pollution prevention in the pulp and paper industry. Compliance with environmental and occupational
safety and health laws is the responsibility of each individual business and is not the focus of this
document. Mention of trade names or commercial products within this report does not constitute
endorsement or recommendation for use. Users are encouraged to duplicate this publication as needed.
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Acknowledgements
This report was prepared by ERG, Inc. of Lexington, Massachusetts under EPA Office of Research
and Development Contract No. 68-CO-0068 for the EPA Office of Pollution Prevention and Toxics. The
OPPT Work Assignment Manager was Jocelyn Woodman, while Jeff Cantin managed the development
of the document for ERG.
The report has been subjected to the U.S. Environmental Protection Agency's peer review process.
The following individuals participated in the review. Their helpful comments are greatly appreciated:
Gary A. Amendola
Amendola Engineering
1052 Kenneth Drive
Lakewood, Ohio 44107
Karl C. Ayers
Director, Env. Programs
Mead Corporation
Courthouse Plaza, N.E.
Dayton, Ohio 45463
Betsy Bicknell
Radian Corporation
2455 Horsepen Road, Suite 250
Herndon, Virginia 22071
Danforth G. Bodien
U.S. EPA Region X
1200 Sixth Ave.
Seattle, Washington 98101
Jens Folke, Managing Director
European Environmental Research
Group, Ltd.
Pinievangen 14
DK-3450 Allerod, Denmark
Steve Geil
U.S. EPA Office of Water
401 M Street SW
Washington, D.C. 20460
David P. Graves
Director, Env. Management
Weyerhaeuser Paper Company
33663 Weyerhaeuser Way South
Federal Way, Washington 98003
George Heath
U.S. EPA Office of Water
401 M Street, SW
Washington, D.C. 20460
K.C. Hustvedt
U.S. EPA Office of Air Quality
Planning and Standards
RTF, North Carolina 27711
Thomas J. Holdsworth
U.S. EPA Office of Research and
Development
26 W. Martin Luther Bang Drive
Cincinnati, Ohio 45268
Anna Klein
U.S. EPA Office of Water
401 M Street, SW
Washington, D.C. 20460
Neil McCubbin
N. McCubbin Consultants
140 Fishers Point
Foster, Quebec
JOE 1RO Canada
Debra Nicoll
U.S. EPA Office of Water
401 M Street, SW
Washington, D.C. 20460
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TABLE OF CONTENTS
Page
SECTION ONE INTRODUCTION 1-1
SECTION ONE REFERENCES 1-5
SECTION TWO POLLUTANTS OF CONCERN IN THE PULP AND PAPER
INDUSTRY . . . . 2-1
2.1 Effluents 2-1
2.1.1 Solids 2-2
2.1.2 Biological Oxygen Demand 2-4
2.1.3 Color 2-6
2.1.4 Chlorinated Organic Compounds . 2-8
2.1.5 Other Toxic Compounds 2-13
2.2 Solid Wastes 2-14
2.2.1 Wastewater Treatment Sludge 2-14
2.2.2 Boiler and Furnace Ash and Scrubber Sludge 2-16
2.2.3 Wood Processing and Other Wastes .... 2-17
2.3 Air Pollutants 2-17
2.3.1 Reduced Sulfur Compounds 2-17
2.3.2 Particulates 2-18
2.3.3 Volatile Organic Compounds 2-18
2.3.4 Chloroform 2-19
2.3.5 Other Hazardous Air Pollutants 2-20
SECTION TWO REFERENCES 2-22
SECTION THREE POLLUTION PREVENTION TECHNOLOGIES IN
WOOD YARD AND CHIPPING OPERATIONS 3-1
3.1 Raw Material Selection . 3-1
3.2 Recycle of Log Hume Water 3-2
3.3 Dry Debarking 3-2
VII
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TABLE OF CONTENTS (cont.)
Page
SECTION THREE POLLUTION PREVENTION TECHNOLOGIES IN
WOOD YARD AND CHIPPING OPERATIONS (cont.)
3.4 Improved Chipping and Screening 3-3
3.5 Storm Water Management ... . . - 3-6
SECTION THREE REFERENCES 3-7
SECTION FOUR POLLUTION PREVENTION TECHNOLOGIES IN
PULPING OPERATIONS 4-1
4.0 Introduction 4-1
4.1 Conventional Kraft Pulping 4-4
4.1.1 Batch Pulping 4-4
4.1.2 Continuous Pulping 4-8
4.1.3 The Kraft Recovery Cycle 4-8
4.2 Extended Delignification 4-10
4.2.1 Number of Installations 4-16
4.2.2 Costs and Economics 4-20
4.2.3 Pollution Prevention Potential 4-21
4.2.4 Compatibility with Downstream Bleaching Stages 4-23
4.2.5 Impacts on Other Aspects of Mill Operations 4-28
4.3 Oxygen Delignification 4-32
4.3.1 Number of Installations 4-39
4.3.2 Costs and Economics 4-42
4.3.3 Pollution Prevention Potential 4-47
4.3.4 Compatibility With Downstream Bleaching Stages 4-47
4.3.5 Impacts on Other Aspects of Mill Operations 4-51
4.4 Ozone Delignification 4-52
4.4.1 Number of Installations 4'57
4.4.2 Costs and Economics 4'60
4.4.3 Pollution Prevention Potential 4'62
4.4.4 Impacts on Other Aspects of Mill Operations 4-64
viii
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TABLE OF CONTENTS (cont.)
Page
SECTION FOUR POLLUTION PREVENTION TECHNOLOGIES IN
PULPING OPERATIONS (cont)
4.5 Anthraquinone Catalysis . . . . . 4-69
4.5.1 Number of Installations 4-69
4.5.2 Costs and Economics ... . 4-70
4.5.3 Pollution Prevention Potential ... 4-70
4.5.4 Impacts on Other Aspects of Mill Operations 4-72
4.5.5 Environmental Effects . . .... 4-72
4.6 Black Liquor Spill Control and Prevention 4-73
4.6.1 Number of Installations . . . 4-75
4.6.2 Costs and Economics 4-75
4.6.3 Pollution Prevention Potential . . . . . 4-75
4.6.4 Impacts on Other Aspects of Mill Operations 4-76
4.7 Enzyme Treatment of Pulp . . 4-76
4.7.1 Number of Installations 4-78
4.7.2 Costs and Economics 4-80
4.7.3 Pollution Prevention Potential 4-81
4.7.4 Compatibility With Other Aspects of Mill Operations 4-81
4.8 Improved Brownstock Washing 4-81
4.8.1 Number of Installations . 4-83
4.8.2 Costs and Economics 4-84
4.8.3 Pollution Prevention Potential 4-84
4.8.4 Impacts on Other Aspects of Mill Operations 4-87
4.9 Closed Screen Room 4-89
4.10 Miscellaneous Pulping Technologies 4-89
4.10.1 The Lignox Process 4-90
4.10.2 Solvent Pulping . . 4-90
4.10.3 Polysulfide Cooking 4-93
4.10.4 Demethylation 4-94
SECTION FOUR REFERENCES 4-96
ix
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TABLE OF CONTENTS (cont)
Page
SECTION FIVE POLLUTION PREVENTION TECHNOLOGIES IN
BLEACHING OPERATIONS 5-1
5.1 Conventional Kraft Pulp Bleaching 5-1
5.2 Chlorine Dioxide Substitution 5-5
5.2.1 Number of Installations 5-10
5.2.2 Costs and Economics 5-14
5.2.3 Pollution Prevention Potential 5-20
5.2.4 Other Impacts 5-24
5.3 Split Addition of Chlorine Charge/Improved pH Control 5-25
5.3.1 Number of Installations 5-26
5.3.2 Costs and Economics 5-26
5.3.4 Pollution Prevention Potential 5-26
5.4 Oxygen-Reinforced Extraction 5-26
5.4.1 Number of Installations 5-28
5.4.2 Costs and Economics 5-28
5.4.3 Pollution Prevention Potential 5-29
5.5 Peroxide Extraction 5-30
5.5.1 Number of Installations 5-32
5.5.2 Costs and Economics 5-34
5.5.3 Pollution Prevention Potential 5-35
5.6 Additional Technology Options in the Bleaching Area 5-36
5.6.1 Improved Chemical Controls 5-37
5.6.2 Improved Chemical Mixing 5-37
5.6.3 Jump-Stage, Counter Current Washing 5-37
SECTION FIVE REFERENCES 5-38
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LIST OF TABLES
Number Page
2-1 Typical Sources and Quantities of Suspended Solids
Generated in Pulp and Papermaking 2-3
2-2 Typical Sources and Amounts of BOD5 Generated in
Kraft Pulp and Papermaking 2-5
2-3 Contribution of Bleaching Stages to Effluent Color
in Conventional Kraft Process 2-7
4-1 Installations of Extended Delignification Systems
Worldwide 4-17
4-2 Characteristics of Conventional and MCC Pulps 4-22
4-3 Impact of Modified Continuous Cooking on Effluent
Characteristics 4-24
4-4 Characteristics of Conventional and RDH Pulping 4-25
4-5 Impact of Rapid Displacement Heating Cooking Techniques
on Effluent Characteristics 4-27
4-6 Sample Boiler Upgrade and Rebuild Projects 4-30
4-7 Typical Operating Data for Oxygen Delignification of Kraft
Softwood Pulp 4-34
4-8 U.S. Installations of Oxygen Delignification Systems 4-41
4-9 Capital Cost Estimates for Oxygen Delignification Systems 4-43
4-10 Potential Oxygen Demand at Bleached Kraft Mill 4-45
4-11 Change in Total Costs for Oxygen Bleaching vs. Conventional
Bleaching in a 600 mt/d Swedish Mill 4-46
4-12 Pollutant Impacts of Oxygen Delignification Versus Conventional
Bleaching 4-48
4-13 Change in Energy Consumption for HC Oxygen Delignification Using
Softwood Kraft at 50% Oxygen Delignification 4-53
XI
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LIST OF TABLES (cont)
Number
4-14 Ozone Pilot and Full-Scale Plants Worldwide ... .............. 4-58
4-15 Bleaching Chemical Costs of Ozone Versus Conventional Sequences at
Union Camp's Franklin, Virginia Mill .......... ....... 4-61
4-16 Emissions from Ozone Bleach Line at Union Camp's
Franklin, Virginia Mill ............. .......... ............ 4-63
4-17 Effluent Properties of Ozone Bleaching Sequences . . .............. 4-65
4-18 Properties of Pulps Produced Using Alternative Bleaching Sequences ..... . . 4-66
4-19 Costs of Anthraquinone Treatment ................................ 4-71
4-20 Major Operating Cost Items for Existing Washing Line Versus
Three Modern Alternatives Hypothetical Mill Retrofit ................... 4-85
4-21 Annual Incremental Operating Costs Saved ($1,000) for Three Modern
Alternative Washing Systems Hypothetical Mill Retrofit ................. 4-86
4-22 Impacts of Improved Washing Practices on Formation of Dioxin ............ 4-88
4-23 Capital Costs for Chiyoda Polysulfide Process ......................... 4-95
5-1 Summary of Bleaching Chemicals ................................ 5-4
5-2 Most Common Bleaching Sequences at U.S. Kraft Mills .................... 5-6
5-3 Summary of Chlorine Dioxide Generation Processes ....................... 5-9
5-4 Levels of Chlorine Dioxide Substitution at U.S.
Kraft Mills .............................................. 5-13
5-5 North American Chlorine Dioxide Generators ........................... 5-15
5-6 Cost and Environmental Comparison of Chlorine Dioxide
Substitution ................................................ 5-17
5-7 Cost and Environmental Comparison of Chlorine Dioxide
Substitution, Greenfield Mill ...................................... 5-18
5-8 Impact of Chlorine Dioxide Substitution Levels on
Chemical Requirements and Costs ................................. 5'19
xii
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LIST OF TABLES (cont.)
Number Page
5-9 Effect of Split Chlorine Addition on Formation of
TCDD and TCDF Formation 5-27
5-10 Impact of Use of Peroxide in Extraction Stages on Chlorine
Consumption and Substitution Rate 5-33
Xlll
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LIST OF FIGURES
Number Page
3-1 Dry Debarking Systems 3-4
3-2 Chip Slicer Showing Oversize Chip Being Split 3-5
4-1 Process Flowsheet for Conventional Integrated Bleached Kraft Mill 4-2
4-2 Batch Digester and Ancillary Equipment 4-5
4-3 Countercurrent Brownstock Washing 4-7
4-4 Continuous Digester and Ancillary Equipment 4-9
4-5 Kamyr Two-Vessel Hydraulic Digester MCC Adaptation 4-13
4-6 Kamyr Single Vessel Hydraulic Digester EMCC Adaptation 4-14
4-7 The Rapid Displacement Heating (RDH) Cycle for Batch
Pulping Systems 4-15
4-8 Process Flows for High Consistency Oxygen Delignification 4-35
4-9 Equipment Diagrams for High-Consistency Oxygen Delignification 4-36
4-10 Process Flows for Medium Consistency Oxygen Delignification 4-38
4-11 Installations of Oxygen Delignification Systems U.S. and
Worldwide 4-40
4-12 Illustration of Typical Wastewater Rows at Bleached Kraft Mill 4-49
4-13 Equipment for High Consistency Ozone Delignification 4-55
4-14 Spill Control System Flow Diagram 4-74
4-15 Hypothesized Reaction of Xylanase with Pulp 4-77
4-16 Equipment Configuration for Xylanase Application 4-77
4-17 Impact of Xylanase Treatment on AOX 4-79
xv
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LIST OF FIGURES (cont.)
Number Page
4-18 Impact of Xylanase Treatment on Brightness 4-79
4-19 Alternative Pulp Washing Equipment 4-82
5-1 North American Consumption of Sodium Chlorate for Chemical
Pulp Bleaching 5-12
5-2 Modification of Extraction Stage for Oxygen Reinforcement 5-31
xvi
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Section One - Introduction Pollution Prevention in Pulp & Paper
SECTION ONE
INTRODUCTION
The Pollution Prevention Act of 1990 asserts that there are significant opportunities for industry
to reduce or prevent pollution at the source, and establishes that pollution should be reduced at the source
whenever feasible.1 In keeping with this national objective, and in an attempt to develop and provide
information on the benefits of source reduction, EPA has produced this report which examines: (1) the
current state of the art, (2) the economics of adoption, and (3) the level of adoption, of selected pollution
prevention technologies in the U.S. pulp and paper industry.
The focus of this report is on the bleached kraft segment of the pulp and paper industry, due to
the heightened concern over its environmental impacts. This concern is related primarily to the use of
chlorine-based compounds in the manufacture of bleached pulps, and the nature of the byproduct
pollutants associated with conventional pulp making processes. In particular, it is the persistence, non-
biodegradability, and toxicity of some of the chlorinated organic compounds formed during chlorine-based
bleaching that explains the high level of attention directed toward this segment of the industry. The
bleached kraft segment accounts for approximately 35 percent of the pulp mills and 47 percent of the pulp
production capacity in the U.S. industry (API, 1992a).
The removal or destruction of chlorinated pollutants from the bleached kraft process through end-
of-pipe treatment is difficult due to their persistence and low concentration in effluents. Conventional
treatment technologies are relatively ineffective in destroying such compounds and instead may result in
their transfer to other environmental media (e.g., wastewater treatment sludge), or even their partitioning
into final products. As a consequence, reduction efforts must focus on changes in the production process
that can reduce or eliminate their formation. The technology options described in this report thus include
a variety of techniques that enable the mill to reduce the use of chlorine-based compounds in the bleaching
process. Because these technologies may enable further recycle of the mill's effluent, they can also lead
to reductions in more traditional pollutants such as biological oxygen demand (BOD5), chemical oxygen
demand (COD), and total suspended solids (TSS), and may further reduce effluent color, water use, and
Public Law 101-508, November 5, 1990.
Page 1-1
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Pollution Prevention in Pulp & Paper Section One Introduction^
sludge volumes generated from the mill's wastewater treatment plant. Emissions of chloroform and other
air pollutants will also decrease as a result of some of these technologies.
Pollution prevention technologies can also further reduce dicharges of non-chlorinated pollutants.
Scientists in Canada and Scandinavia have recently suggested that such non-chlorinated substances can
make a significant contribution to the effects of (treated) pulp mill wastes on the receiving waters (see
Section 2.1.5).2 By increasing the volume of effluent recycled through the recovery boiler, most pollution
prevention technologies reduce the discharges of such non-chlorinated substances to the treatment system
and receiving waters.
The economics of adopting process changes are explored in detail in this report. It is important
to note that, while it is possible to cite representative capital and operating cost information, the actual
costs and savings for any particular mill are very site-specific, and depend closely on the age, type, and
condition of the existing equipment at the mill. A key consideration affecting the attractiveness of any
of these options is the relative age and obsolescence of equipment it will replace, and the future
investments that may be avoided as a consequence of adopting in-plant pollution prevention measures.
Additional savings in the form of reduced or avoided treatment and compliance costs and, potentially,
exposure to liability from pollution-related litigation may also factor into the decision to adopt prevention
technologies.
The costs presented in this report are for specific examples drawn from the literature for the
purposes of putting the economic aspects in perspective. Due to the wide variation in situations among
mills, it is recommended that evaluations of these technologies for a particular mill be based only on site-
specific engineering reports that clearly identify the scope of the project, detail the necessary capital
equipment and operating costs, and that are explicit with regard to any savings assumed to accrue.
In general, pollution prevention technologies in kraft pulping and bleaching result in higher capital
but lower operating costs for the mill. Using conventional project evaluation techniques, in-plant
prevention measures may not generate sufficient savings to justify the investment costs themselves. The
decisionmaking process at an individual mill, however, can be substantially affected by the market and/or
regulatory environment it expects to face in the future. Many mills are undoubtedly concerned about the
2 At this time, only limited information is available concerning these findings, although further results
are expected to be published shortly.
Page 1-2
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Section One - Introduction Pollution Prevention in Pulp & Paper
future direction of environmental regulations in their industry and the possible implications on the
processes they use. Market forces are equally important. In particular, mills that sell pulp or paper into
certain environmentally discerning international markets may be forced to adopt further pollution
prevention measures in order to comply with the demands of their customers for "environmentally
responsible" paper and pulp products.
One factor to consider when evaluating the viability of pollution prevention technologies is that
operating costs may be sensitive to the target pulp brightness level. This is especially true in totally
chlorine-free (TCP) processes, which may use expensive hydrogen peroxide in the final bleaching stage
to bring pulp to final brightness. The higher the producer's brightness requirements, the more peroxide
must be used, and the higher the bleaching costs.
Traditionally, mills that produce "market pulp" for sale to other mills have had to meet higher
brightness standards than most integrated mills - mills that produce pulp for their own use in papermaking
- require. Pollution prevention technologies involving non-chlorine bleaching stages are more competitive
with conventional processes in the 70 to 80 brightness range.3 Unless market pulp brightness levels fall,
therefore, integrated mills that can use lower brightness pulps will be better positioned than market pulp
producers to take advantage of some of the pollution prevention technologies discussed in this handbook.4
This may become significant since market pulp producers in the U.S. sell much of their product to
European customers, who are increasingly looking at the processes used to manufacture the pulps they
buy.5
3 Most pulp mills have traditionally applied bleaching chemicals to achieve a target brightness level
of 88 to 90 percent ISO. In particular, market pulp (i.e., pulp sold to other mills for use in papermaking)
has always been bleached to high brightness according to the demands of pulp buyers. Many integrated
mills (i.e., nulls that produce pulp for their own use in papermaking) are able to make quality paper
products using pulp bleached to somewhat lower brightness levels (between 80 and 88 percent, depending
on the source). Brightness targets above 85 percent ISO are both technically more difficult and
substantially more costly to achieve using alternative and emerging technologies. For further discussion
of pulp brightness, see Section 5.1.
4 The market issues surrounding pulp brightness and pollution prevention are addressed in several
papers contained in the proceedings from the EPA-sponsored International Symposium on Pollution
Prevention in the Manufacture of Pulp and Paper - Opportunities and Barriers (EPA, 1993).
5 In 1991, U.S. exports of paper grade wood pulp to Western Europe totalled 2.1 million metric tons
(or 41.3 percent of the total) (API, 1992b).
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Pollution Prevention in Pulp & Paper Section One Introduction
Much of the information contained in this report is by necessity very recent. Many of the current
concerns over the environmental problems of the U.S. pulp and paper industry have arisen only since
1985, with the discovery of dioxin in bleached kraft mill effluents and solid wastes (EPA, 1988).
Although prior to 1985 some of these alternative and emerging technologies were in use elsewhere in the
world (and were under active investigation in North America), only lately has there been a move by U.S.
producers to adopt them. Since the discovery of dioxins in pulp mill effluent, however, the U.S. and
international research and development effort has been impressive, and the rate of adoption of many of
these in-process pollution prevention technologies has been increasing rapidly. Information on their use,
effectiveness, and cost has been spreading through all of the major trade publications and at numerous
industry conferences. As experience with the technologies grows, it is inevitable that costs will decline
and effluent will further improve, providing additional incentives for adoption.
This report is organized into four sections. Section Two covers the primary pollutants of concern
in the pulp and paper industry. This section provides background for discussion in further sections on
technologies that reduce these pollutants. Sections Three, Four, and Five cover pollution prevention
technologies that are available to reduce or minimize the generation of some of these pollutants. Section
Three covers technologies that can be applied in the woodyard and chipping areas of the mill. Section
Four addresses technologies associated with the pulping or pre-bleaching stages of the process, while
Section Five deals with alternative bleaching technologies. The first parts of Sections Four and Five
include information on the conventional processes used in kraft pulping and bleaching to facilitate
discussion of alternative techniques.
It should be noted that in addition to the pollution prevention technologies presented in this report
there are numerous additional technologies, not necessarily meeting the definition of pollution prevention,
that may be of interest to some readers. These include water conservation, solid waste reduction, and
treatment technologies that can be applied in the woodyard, pulping, bleaching, and paper-making areas
of kraft mills. Further information concerning these technologies can be found in a separate EPA report
(EPA, 1992).
Page 1-4
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Section One Introduction Pollution Prevention in Pulp & Paper
SECTION ONE REFERENCES
API, 1992a. American Paper Institute. 7992 Statistics of Paper, Paperboard, & Wood Pulp, New York.
API, 1992b. American Paper Institute. Exports of Pulp, Paper, Paperboard and Converted Products to
World Markets 1991. International Department. New York.
EPA, 1988. U.S. Environmental Protection Agency. U.S. EPA/Paper Industry Cooperative Dioxin
Screening Study. Office of Water Regulations and Standards, Washington, D.C., March 1988.
EPA 440-1-88-025.
EPA, 1992. U.S. Environmental Protection Agency. Model Pollution Prevention Plan for the Kraft
Segment of the Pulp and Paper Industry. U.S. EPA Region 10, Seattle, WA, September 1992.
EPA 910/9-92-30.
EPA, 1993. International Symposium on Pollution Prevention in the Manufacture of Pulp and Paper -
Opportunities and Barriers, August 18-20, 1992, Washington, D.C. U.S. Environmental
Protection Agency, Office of Pollution Prevention and Toxics. EPA-744R-93-002. February
1993.
Page 1-5
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Section Two - Pollutants of Concern Pollution Prevention in Pulp & Paper
SECTION TWO
POLLUTANTS OF CONCERN IN THE PULP AND PAPER INDUSTRY
This section discusses the sources, types, and quantities of pollutants found in the waste streams
of bleached kraft pulp and paper mills, and me methods currently in use for their control.
Section 2.1 describes effluent discharges, including conventional pollutants, toxics and, in
particular, chlorinated organic compounds. Section 2.2 discusses the solid wastes, while Section
2.3 discusses air emissions.
2.1 EFFLUENTS
Pulp and paper mills require large quantities of water for wood handling, pulping, washing,
bleaching, and papermaking operations. Water consumption has declined considerably over the past three
decades, however, as mills have initiated water reuse programs to "close up" the process water flow. For
example, in 1959 the U.S. pulp and paper industry consumed 57,000 gallons of water per ton of
production. By 1988, this figure had dropped to 16,000 to 17,000 gallons per ton (Miner and Unwin,
1991). Nevertheless, at these rates a 600 tpd mill still requires approximately 10 million gallons of
influent water per day, and must treat and discharge approximately the same amount (net of evaporative
losses).
Effluent guidelines for the pulp and paper industry were first promulgated in November, 1982.
The main categories of aquatic pollutants addressed hi these effluent guidelines were: suspended solids,
biochemical oxygen demand (BOD5), color, and toxics.1 Conventional pollution abatement in the U.S.
has concentrated on reducing solids, oxygen demand, and aquatic toxicity. Color has been perceived as
a problem only in isolated instances, and until late has not received significant regulatory attention at the
national level.2 Recent investigations have found toxic contaminants in bleach mill effluents that were
1 The November 18, 1982 effluent limitations guidelines (47 FR 52006) established limits for the
conventional pollutants BOD5, TSS, and pH, and for the priority pollutants zinc, pentachlorophenol, and
trichlorophenol.
2 State water criteria and standards have addressed color in some localities, and these requirements
will be reflected in the NPDES permits of affected mills.
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Pollution Prevention in Pulp & Paper Section Two_
not confirmed prior to 1985 (e.g., dioxins). These pollutants are now included in the current (scheduled)
revisions of effluent guidelines for the industry under the Clean Water Act.
2.1.1 Solids
Solids consist of both suspended and dissolved materials carried in the effluent stream. In a
conventional integrated kraft mill, the solids load in untreated effluent consists mainly of: (1) dirt, grit,
and fiber from the wood preparation stages, (2) screen rejects and spills from the pulping area, (3) fiber
and dissolved lignin solids from the pulp bleaching stages, and (4) fiber and additives washed from the
early stages of papermaking.
Virtually all U.S. mills have installed primary and secondary effluent treatment designed, in part,
to remove solids from the effluent before it is discharged to the receiving waters. Suspended solids are
normally removed by settling or flotation processes that take place during primary wastewater treatment.
Dissolved solids not removed by primary treatment are subjected to the biological processes that occur
during secondary wastewater treatment.3 Some of the inorganic (mineral) fraction of the suspended solids
pass through both primary and secondary processes and are discharged with the final effluent. Typical
quantities of suspended solids produced at various pulp- and papermaking stages are shown in Table 2-1.
In the past, the release of settleable suspended solids in pulp and paper mill effluents was
significant, and posed an environmental hazard after their release to the receiving waters. These particles
can blanket the bottom of the receiving waterbody and destroy the habitat of bottom-living organisms.
As the solids blanket decomposes, anoxic conditions may develop, resulting in the release of methane,
hydrogen sulfide, and other noxious and/or toxic gases. In extreme cases, suspended fibers can also be
lethal to fish. Nowadays, well-operated primary treatment systems are capable of removing most of the
3 Dissolved organic solids are associated with the effluent's biochemical oxygen demand, and are
addressed in the following section.
Page 2-2
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Section Two Pollutants of Concern
Pollution Prevention in Pulp & Paper
TABLE 2-1
Typical Sources and Quantities of Suspended Solids
Generated in Kraft Pulping and Papermaking
Source
Wood Yard
Pulping
Recovery
Bleaching
Papermaking
Solids Contribution
(Ib/ton)
1 to 100
Oto60
5 to 10
2 to 6
10 to 60
Key factors
Dry vs. wet debarking; use of log
flumes.
Amount of screen rejects sewered;
whether screen room is closed; level of
spill controls.
Amount of lime mud sewered; fate of
liquor grits; condensate stripping and
fate of condensates.
Intra-stage washing efficiency.
Save-all efficiency; paper grade (amount
of fillers, sizing, etc. added).
Source: Various industry observers.
Page 2-3
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Pollution Prevention in Pulp & Paper Section Two_
settleable solids. Concern remains, however, because heavy metals, dioxins, and other chlorinated and
unchlorinated compounds tend to adsorb to any remaining particles.
2.1.2 Biochemical Oxygen Demand
Biochemical oxygen demand (BOD5) is a measure of the tendency of an effluent to consume
dissolved oxygen from receiving waters.4 The consumption of oxygen results from natural biochemical
degradation that occurs as complex organic materials are consumed by microorganisms present in the
water. High levels of BOD5 in the effluent stream can deprive nonphotosynthetic organisms (i.e., fish,
shellfish, fungi, aerobic bacteria) of the oxygen they need to survive. BOD5 has been used as a generic
term for all organic material because organic compounds are the substrate responsible for the measured
oxygen demand. Any regulation or procedure which reduces BOD5 will thus reduce the total organic
content of the water as well.
High-BOD5 effluent is produced at many stages throughout the pulping and bleaching processes,
including: debarking, washing, cooking, condensing of spent liquors, and bleaching. The BOD5 in effluent
from wet drum or hydraulic debarking is associated with wood particles and dissolved organics that remain
in the wash water after the logs are stripped. Dry debarking generates no effluent load at this stage, but
results in higher BOD5 levels from pulping operations (because more bark remains on the logs, requiring
a higher volume of cooking liquor). Spent cooking liquor (weak black liquor) contains much of the lignin
and other organic materials originally contained in the wood. The weak black liquor is concentrated and
routed to the recovery system, however, where much of the BOD5-causing substances are incinerated.
Digester condensates and condensates from weak black liquor concentration may contain up to
one-third of the untreated wastewater loadings BOD5 at bleached kraft mills. Chlorination and extraction
stages generate BOD5 during bleaching operations; this BODS is associated with dissolved lignin, other
carbohydrates, and fiber that is dissolved during bleaching. Typical quantities of BOD5 produced during
pulp and paper production are shown in Table 2-2.
4 BOD5, representing the 5-day biochemical oxygen demand of effluents, is the most common
pollutant parameter used in the U.S. and will be used throughout the remainder of this report.
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Section Two Pollutants of Concern
Pollution Prevention in Pulp & Paper
TABLE 2-2
Typical Sources and Amounts of BOD5
Generated in Kraft Pulping and Papermaking
Source
Wood Yard
Pulping
Recovery
Bleaching
Papermaking
BOD Contribution
Ib/ton
Oto 10
OtoSO
2 to 20
3 to 40
5 to 30
Notes
Dry vs. wet debarking,
flumes.
use of log
Level of spill control
Level of spill control
Amount of bleach plant
be recycled to recovery
bleach sequence)
effluent that can
(depends on
Type of product (amount of additives).
Source: Various industry observers.
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Pollution Prevention in Pulp & Paper Section Two
During secondary treatment of effluent, most BOD5 is removed. In an oxidation lagoon, a 30-day
retention period removes 85 to 90 percent of BOD5, while an aerated lagoon requires a fraction of that
time to produce similar results.
2.13 Color
"Color" is a measure of an effluent's interference with the transmission of light. Because it
reduces light levels in receiving waters, high doses of color can disrupt photosynthesis and aquatic life.
The primary concern over effluent color is its undesirable aesthetic effect on receiving waters. The
compounds contributing to color are also associated with water taste problems and can stabilize some
bivalent metal ions by chelation.5 Although materials that impart color to mill effluent are generally
nontoxic, and are not known to cause harm to the receiving waters (except at very high loadings), their
impact on the aesthetic qualities of some waterways has led to increased regulatory attention at the local
level.6
High molecular mass materials, primarily dissolved lignin and lignin derivatives, hold the bulk
of the chromophores (color bodies) present in pulp and paper effluent. The molecules responsible for
color break down slowly in the aquatic environment, eventually reaching a size small enough to be
incorporated into microbial metabolism. This process is reflected in a long-term biochemical oxygen
demand over a period of 20 to 100 days or longer, which is not measured by the conventional BOD5 test.
Table 2-3 indicates that over half of the color load in kraft pulp mill effluents comes from the first
caustic extraction stage in the bleach plant; most of the remainder is generated during the first chlorination
stage. As noted in the table, color is usually measured in "platinum cobalt units" (PCU), expressed as
pounds PCU per ton of pulp. Effluent from a 1970s-era integrated CEDED softwood kraft pulp mill may
contain approximately 300 Ibs of color per ton pulp (hardwood pulping contributes less than half this
amount). Of this total, pulping contributes about 20 percent and the bleach plant contributes about 70
5 Technically, taste problems are caused by the pollutants that cause color. It follows that actions
taken to reduce color will also reduce taste problems as well.
6 Color regulations are normally established based on narrative criteria contained in the facility's
NPDES permit (e.g., "no significant impact on receiving water color").
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Section Two Pollutants of Concern
Pollution Prevention in Pulp & Paper
TABLE 2-3
Contribution of Bleaching Stages to
Effluent Color in Conventional Kraft Process
Bleaching Stage
1 C - Chlorination
2 E Caustic extraction
3 D Chlorine dioxide
4 E Caustic extraction
5 D Chlorine dioxide
TOTAL
Softwood
kg/ADT
50
226
11
6
1
294
% of Total
17.0
76.9
3.7
2.0
0.3
100.0
Hardwood
kg/ADT
26
78
6
4
1
115
% of Total
22.6
67.8
5.2
3.5
0.9
100.0
Note: Units are Am. Pub. Health Assoc. (APHA) chloroplatinate units, kg/ton.
For further details on bleaching chemicals see Section 5.
Source: Ontario Ministry of the Environment (1988). Data based on Rush and Shannon (1976).
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Pollution Prevention in Pulp & Paper Section Two
percent. The remainder is generated from a number of minor sources, including wood preparation,
chemical recovery, and paper-making operations. Within the bleach plant, caustic extraction is the largest
single source of color, contributing some two-thirds of the color in bleach plant effluents, or nearly half
of all color generated at an integrated mill (Springer 1986).
Conventional biological treatment removes less than 10 percent of the effluent color (Ho et al.,
1991). Although water quality standards in many states address color, color standards are currently
included in the NPDES permits of only five pulp and/or paper mills (Geil, 1993).7'8 Where it is
necessary, some mills employ a separate clarification stage to remove effluent color. Most pollution
prevention technologies discussed later in this report will reduce color substantially.
2.1.4 Chlorinated Organic Compounds
Discharges of chlorinated compounds are associated almost exclusively with bleach plant
operations at pulp mills that use elemental chlorine or chlorine-containing bleaching chemicals.
Discharges may also occur from paper mills (or papermaking operations at integrated mills) which use
chlorine-bleached pulps, but these discharges are small in comparison.
Terminology, Units of Measurement, and Test Methods
A number of alternative test methods have been developed and/or adapted to yield quantitative
estimates of chlorinated compounds in pulp and paper mill effluents. This is an active area of research,
as scientists attempt to develop methodologies that accurately reflect the biological activity and potential
impacts of the many classes of chlorinated compounds that are present in these effluents. The most
common tests include the following:
7 State water quality criteria generally do not specify color limits; rather they include language
requiring, for example, that effluent be "free" of color. It is then up to the NPDES permit writer to
determine whether the mill's discharge permit should address color, including whether monitoring and
reporting of color levels is required.
8 A further ten mills have standards for turbidity (cloudiness) which, according to Geil (1993), may
be a surrogate for color.
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Section Two Pollutants of Concern Pollution Prevention in Pulp & Paper
Total Organic Chlorine (TOC1) This test measures all organically bound chlorine in an
effluent (or other) sample. The sample is passed through one or more media in which organic
compounds are captured. The medium is combusted to destroy all organics, and chlorine is
captured and measured from the combustion byproducts. Chlorine concentration is expressed as
a proportion of the initial sample (e.g., g chlorine per kg pulp, kg chlorine per metric ton pulp).
Total Organic Halogens (TOX) Identical to TOC1 in procedure, except that all of the halogens
(fluorine, bromine, and iodine, in addition to chlorine) are measured. Results are typically nearly
identical to TOC1 results, because only traces of fluorine, bromine, and iodine are generally
present in effluents.
Adsorbable Organic Halogens (AOX) - Conceptually very similar to TOC1 or TOX, except that
organics are adsorbed onto granular activated charcoal in the initial step. The primary
advantage of this test is that it can be completed much more rapidly than TOC1. Results are also
expressed as the proportion of chlorine to total sample weight (e.g., g chlorine per kg pulp, kg
chlorine per metric ton pulp), and are generally highly correlated with TOC1 results.
Extractable Organic Halogens (EOX) - Procedurally identical to AOX, except that effluents are
first extracted with a nonpolar solvent. EOX compounds include those that can be expected to
be lipophilic, i.e., to show a tendency to bioaccumulate in the fatty tissues of living organisms.
Historically, TOC1 has been most often used to express the total organic chlorine content of pulp
and paper mill effluents and other wastes. In recent years, however, AOX has become a more or less
standard measure. Most regulatory requirements in European countries and Canada are based on the AOX
measurement, and AOX monitoring requirements have begun to be incorporated into some NPDES permits
in the United States. It has been suggested however, that EOX or other measurements that are more
closely correlated with biologically active chlorinated compounds should be used as the basis for
regulating chlorine discharges (e.g., Folke et al., 1991; MacKay, 1989), and research toward this end
remains active.
Organic chlorine in effluents can also be estimated as a function of the amount of chlorine used
in the bleaching process. Germgard (1983) proposed the following relationship:
TT rj
Organic chlorine = k*\C+—+—} kg/metric ton pulp
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Pollution Prevention in Pulp & Paper Section wo_
Where:
C, H, and D . represent the molecular chlorine, hypochlorite and chlorine dioxide
charges in kg per tonne pulp, (with H and D expressed as equivalent
molecular chlorine); and
k equals a constant in the range of 0.07 to 0.15.
Liebergott (1991) has since shown that the values for k depend on the level of chlorine dioxide
substitution. To estimate AOX, McCubbin et al. (1992) add the factor (l-eB) to the equation, and adjust
k depending on the substitution level:
ff D
AOX = k* {C+—+—}*{ 1 -es} kg/metric ton pulp
Where:
Cj, equals the AOX removal efficiency of the biological treatment system (40
percent for aerated stabilization and 33 percent for activated sludge);
k equals 0.08 when the bleach plant operates with less than 70 percent
chlorine dioxide substitution; and
k equals 0.08 x [1.7 (%subst. -=- 100)] where substitution is greater than
70 percent.
Identified Chlorinated Organic Compounds
More than 300 individual chlorinated organic compounds have been identified to date in bleach
plant effluents. The major classes of compounds that have been identified include: chlorinated acids,
chlorinated phenolics, chlorinated aldehydes, ketones, and lactones, and chlorinated hydrocarbons.
These compounds contain only a small fraction of the total mass of chlorine contained in effluents,
however. By far the larger proportion of all organically bound chlorine (75 to 90 percent) is incorporated
into very large molecules (molecular weight > 1,000), many of which have not been specifically
characterized. These molecules typically consist of •chlorinated fragments of complex lignin species, which
are not amenable to precise characterization. Because of their large size, such high molecular weight
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Section Two Pollutants of Concern Pollution Prevention in Pulp & Paper
compounds are difficult to break down in treatment. The extent of their breakdown during treatment, the
nature of possible breakdown products, and the potential magnitude of their related environmental impacts
has been extensively investigated, though few conclusions have been reached.
Although many of the chlorinated compounds identified in pulp and paper mill effluents have
some potential to damage living systems, to date only a small number have been the subject of specific
scientific and regulatory attention.9 Those which have been widely studied include dioxins and furans,
chloroform, and chlorates; they are addressed individually in the following paragraphs.
Dioxins and furans Dioxins and furans (specifically, dibenzo-p-dioxins and furans) are a class
of chlorinated organic compounds that contain two aromatic carbon rings joined by a bridge of carbon-
carbon and carbon-oxygen bonds. A total of 75 dioxins and 135 furans have been identified. The
biological activity and impacts of these compounds depend on the number of chlorine molecules attached
to the double ring structure and their location on the dioxin/furan molecule. The most toxic are 2,3,7,8-
TCDD (2,3,7,8-tetrachloro-dibenzo-p-dioxin) and 2,3,7,8-TCDF (2,3,7,8-tetrachloro-dibenzo-furan), both
of which contain four chlorine molecules.
Like all of the chlorinated organics found in pulp and paper effluents, dioxins and furans are not
formed as a planned product of the bleaching process, rather they are a byproduct generated by the
chlorination of nonchlorinated precursors during the complex reactions that occur during bleaching. The
concentration of these chemicals is not great - even in uncontrolled effluents, concentrations are typically
measured in parts per trillion to parts per quadrillion. Concern has arisen from their high toxicity,
persistence, and potential for bioaccumulation rather than high effluent concentrations per se. Lignin
contains chemical structures from which dioxins may be generated, and is thought by some to provide
most or all of the precursors associated with dioxin formation. Other sources have been suggested,
including natural compounds in mill influents (Ontario Ministry of the Environment, 1988), and oils used
as defoamers during pulping and bleaching (Berry et al., 1989).10
9 Many individual compounds have been studied on a collective basis (e.g., AOX, BOX, TOC1).
10 In the last several years the replacement of precursor-containing defoamers has caused a significant
decrease in dioxin production.
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Pollution Prevention in Pulp & Paper Section Two_
Secondary wastewater treatment is moderately effective at removing dioxins and furans from
effluents, and developing technologies have promised removal efficiencies of greater than 90 percent
(OTA, 1989). Wastewater treatment does not, however, result in the destruction of dioxins and furans,
but simply in their transfer to treatment sludges, where disposal remains a significant problem. It is for
this reason that scientific and regulatory attention has generally focused on technologies that prevent the
formation of dioxins and furans during bleaching, and not on biological or chemical effluent treatment.
The U.S. industry has also recognized the role of pollution prevention techniques in reducing dioxin and
furan formation. According to estimates from the National Council of the Paper Industry for Air and
Stream Improvement (NCASI),11 more than $2 billion has been spent on water pollution control in the
U.S. since 1985, much of this aimed at reducing dioxins and furans (API, 1992).
Chloroform The hypochlorite bleaching stage (used in a large but decreasing number of mills)
is by far the major source of chloroform generated during pulp bleaching. The chlorination and extraction
stages are also associated with chloroform generation, and are the major sources when a hypochlorite stage
is not used. A number of factors influence chloroform generation, the most important being the amount
of hypochlorite used and the variation hi pH during bleaching and extraction. Chlorine dioxide
substitution generally results in a reduction in chloroform formation (because it replaces hypochlorite), as
does a reduction in chlorination stage temperature (although this is not a useful control parameter)
(Dallons et al., 1990; Crawford et al., 1991).
Because chloroform is an extremely volatile compound, 60 percent or more of all discharges are
typically released as fugitive air emissions through bleach plant vents (Dallons et al., 1990; see also
Section 2.3.4). In plants that employ secondary wastewater treatment, as much as 80 percent of the
remaining chloroform may escape as fugitive emissions from water treatment facilities (aeration basins,
aerators, primary and secondary clarifiers) (Ontario Ministry of the Environment, 1988). The remainder
will volatilize gradually following release to the receiving waters. Because its aquatic toxicity and
bioaccumulation potential are low, chloroform in pulp and paper effluents is not considered to be a
significant aquatic hazard. However, it is considered both a toxic pollutant under the Clean Water Act
and a hazardous air pollutant (HAP) under the Clean Air Act, and will be subject to regulation under the
upcoming integrated pulp and paper rulemaking.
11 NCASI is the environmental arm of the American Forest Products Association (AFPA), formerly
the American Paper Institute (API).
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Section Two Pollutants of Concern Pollution Prevention in Pulp & Paper
Chlorate Chlorate (ClOj) is a by-product formed during chlorine dioxide bleaching. The
quantity of chlorate produced is insignificant in most mills, but up to 3 kg per ton of pulp can be
generated and found in untreated wastewater when high rates of chlorine dioxide substitution are used
(Germgard, 1988). Chlorates are a potential concern because some compounds are known to harm plant
life; for example, sodium chlorate has been used as an herbicide for weed control. At concentrations
found in untreated effluents, chlorate damage to marine algae populations in Sweden has been documented
(e.g., Germgard, 1988). Chlorate is effectively removed from effluents during secondary treatment
providing they have an anoxic section installed deliberately (or otherwise), and similar biological processes
also remove chlorate from natural receiving waters. Since U.S. mills have only recently begun using high
substitution bleaching, there are no reports of damage to freshwater species so far. At the present time,
chlorates in pulp and paper effluents do not pose a serious threat to the environment, but with higher rates
of chlorine dioxide substitution they may become a focus of concern in the future.
2.1.5 Other Toxic Compounds
Resin acids and fatty acids are pulping byproducts which form a "soap" that is skimmed from the
pulp during the recovery process. About eighty kilograms of soap are produced per ton of pulp. This
soap is generally incinerated, though in some cases it is captured for processing into tall oil (sold as a
pulping byproduct). Occasionally, the soap causes foam overflows, which can escape into the effluent
stream. The environmental effects from soap spills can be severe, since these compounds are acutely toxic
to many aquatic species. Their impact can be felt on both the biota of receiving waters and on the
populations of bacteria and fungi that are responsible for biological wastewater treatment.
Recently, scientists in Canada and Scandinavia have suggested that non-chlorinated substances
represent a large portion of the remaining toxicity of effluents from mills that have reduced discharges
of chlorinated organics to below traditional levels (Lehtinen, 1991; MFG, 1991; Van der Krakk et al.,
1992). Lehtinen, for example, has concluded that there is no correlation between the amount of AOX
formed during bleaching and the composite biological response of fish retained in dilute mill effluents
(Lehtinen, 1991). In other studies, the effluent from unbleached kraft mills showed a stronger response
than that obtained from mills producing up to 4 kg per ton AOX (Brunsvik, 1991; Ladner, 1991). While
these findings are preliminary, it has been hypothesized that the biological effects are due to steroids
present in wood extractives, which may not be eliminated during secondary treatment. In-plant measures
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Pollution Prevention in Pulp & Paper Section Two
that would reduce the total discharge of organics would have the added benefit of reducing the amount
of such non-chlorinated substances discharged from the mill.
2.2 Solid Wastes
The significant solid waste streams from pulp and paper mills include bark, wastewater treatment
sludges, lime mud, lime slaker grits, green liquor dregs, boiler and furnace ash, scrubber sludges, and
wood processing residuals. Because of the tendency for chlorinated organic compounds (including
dioxins) to partition from effluent to solids, wastewater treatment sludge has generated the most significant
environmental concerns for the pulp and paper industry. To a lesser extent, concern has also been raised
over whether chlorinated organics are partitioned into pulp products, a large portion of which becomes
a post-consumer solid waste. This section discusses disposal of wastewater treatment sludge, scrubber ash
and sludge, and wood residues
2.2.1 Wastewater Treatment Sludge
With the exception of bark, wastewater treatment sludge is the largest volume solid waste stream
generated by the pulp and paper industry. Pulpmaking operations are responsible for the bulk of these
wastes, although treatment of papermaking effluents also generates significant sludge volumes. For the
majority of pulp and integrated mills that operate their own wastewater treatment systems, sludges are
generated onsite. A small number of pulp mills, and a much larger proportion of papermaking
establishments, discharge effluents to publicly-owned wastewater treatment works (POTWs). Sludges
associated with these mills are generated at public facilities, where they form a portion of total sludge
generated from mixed industrial, commercial, and residential sewage.
Wastewater treatment sludges themselves do not pose a significant environmental concern.
Potential environmental hazards are associated with trace constituents (e.g., chlorinated organic
compounds) that are partitioned from the effluent. The 1988 results of the "104-Mill Study" showed that
dioxins and furans were present in bleached pulp mill sludges, resulting in calls to regulate both landfill
disposal and land application of such sludges. Landfill and surface impoundment disposal are most often
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Section Two Pollutants of Concern Pollution Prevention in Pulp & Paper
used for wastewater treatment sludge; in 1988 only eleven of 104 bleached kraft mills disposed of any
sludge through land application or conversion to sludge-derived products (e.g., compost, animal bedding).n
Sludge generation rates very widely among mills. For example, bleached kraft mills surveyed as
part of EPA's 104-Mill Study reported sludge generation that ranged from 14 to 140 kg sludge per ton
pulp (EPA, 1988). Total sludge generation for these 104 mills was 2.5 million dry metric tons per year,
or an average of approximately 26,000 dry metric tons per year per plant.
Two different types of wastewater treatment sludge are generated at the mill, each of which
exhibits very different physical and chemical characteristics. Primary sludge includes floating and
suspended solids that are removed from the pulp by the physical processes (e.g., screening, skimming,
sedimentation, flotation) used during primary wastewater treatment. Primary sludge may also contain
chemical coagulants and/or flocculants that are added to effluents to promote settling of suspended solids.
Primary sludge therefore consists primarily of unaltered constituents of the wood that is input to the pulp
and papermaking process — bark and other wood residuals, knots and other rejects that enter pulping
effluents, paper machine additives (clays, fillers), and fiber that is lost from pulping, washing, and
bleaching operations.
The solids in secondary sludge consist almost exclusively of bacterial and fungal biomass that is
generated during biological treatment of dissolved and suspended organic matter in wastewaters, including
bleach plant effluents. Pollutants that are sequestered in these organic compounds tend to become
concentrated in secondary sludges; it is for this reason that secondary sludges have become a focus of
12 Under the terms of a 1988 consent decree (EDF/NWF v. Thomas, D.D.C. No. 85-0973, July 27,
1988) EPA announced in November, 1991 their finding that there was insufficient evidence of potential
risk to justify regulation under the Resource Conservation and Recovery Act (RCRA) of landfill or surface
impoundment disposal of bleached pulp and paper mill sludge. Under a separate consent decree, (EOF
v. Reilly, D.D.C. No. 89-0598) EPA is required to "...promulgate a listing determination for sludges from
pulp and paper mill effluent on or before the date 24 months after promulgation of an effluent guideline
regulation under the Clean Water Act for pulp and paper mills," (p. 10). The decree specifies, however,
that a listing determination would not be required if the final rule for the effluent guideline revision is
based on "...the use of oxygen delignification, ozone bleaching, prenox bleaching, enzymatic bleaching,
hydrogen peroxide bleaching, oxygen and peroxide enhanced extraction, or any other technology involving
substantially similar reductions hi uses of cMorine-containing compounds," (pp. 10-11).
Also in response to the 1988 consent decree, regulatory actions to control land application under
the Toxic Substances Control Act (TSCA) were proposed hi April, 1991 and are still under development.
Since that time, dioxin levels hi mill effluent and, presumably in sludges, have declined considerably.
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Pollution Prevention in Pulp & Paper _ Section Two
environmental concern. The largest volumes of secondary sludges are generated by activated sludge
treatment systems, which aggressively promote biomass growth and turnover. Much smaller volumes are
generated by aerated lagoon treatment systems, as a large percentage of the solids in these systems are
aerobically and anaerobically destroyed.
Additional sources of wastewater treatment sludge include coagulation and/or flocculation
treatment stages designed to capture specific pollutants. For example, lime or alum coagulation is used
by a few pulp mills to control color discharges, and generates a sludge that must be collected and
disposed. Depending on specific treatment process design, these sludges may be captured as a portion
of primary sludge or may be collected in an independent step.
2.2.2 Boiler and Furnace Ash and Scrubber Sludge
The power boiler and recovery furnace are the major sources of ash from pulpmaking operations,
while the lime kiln is a secondary and relatively minor source. Two different types of ash are generated
by the combustion processes at pulp and paper mills. Fly ash consists of fine particles that are entrained
in and subsequently captured from flue gases by emission control devices, while bottom ash consists of
coarse noncombustible particles that are removed continuously or periodically from boiler and furnace
combustion chambers. Although the volume of ash generated depends to some extent on boiler/furnace
design and operating conditions, ash generation is primarily a function of the fuels that are consumed.
Coal and wood fuel generate the largest volumes of ash, whereas liquid and gaseous fuels produce very
little or no ash.
Scrubber sludges are also associated with ash generation. A number of mills use wet scrubbers
to capture particulate (and occasionally other) emissions. Sludges generated by these scrubbers contain
the captured particulate or gaseous pollutant species. The disposal of ashes and scrubber sludge is a
concern in the industry, due to the generally low pH of these wastes. Many mills must raise the pH by
mixing ash and scrubber sludges with Lime, bark, or wood chips. This increases the bulk of the waste and
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Section Two Pollutants of Concern Pollution Prevention in Pulp & Paper
increases the cost of their disposal. Fly ash from hog fuel burners is also a concern due to the presence
of dioxins and furans in the ash at some mills.
2.2.3 Wood Processing and Other Wastes
A number of other minor solid waste streams are associated with pulp and paper operations. Dry
wood preparation operations and groundwood pulping generate significant volumes of residuals. To the
maximum extent practicable, however, these wastes are captured and consumed as fuel in power and/or
steam boilers. Knots, bark, and other pulping rejects may be captured as a solid waste stream separate
from effluent flows. Mills also generate significant quantities of mixed industrial solid wastes including
pallets, chemical shipping containers, construction debris, and other items.
2.3 Air Pollutants
The major air pollutants from kraft pulping and bleaching operations include reduced sulfur
compounds, particulates, hazardous air pollutants (HAPs), including methanol and chloroform, and volatile
organic compounds (VOCs), some of which may also be HAPs. SOX, SO2 and, to a lesser extent NOX are
also a concern. The following sections discuss emission sources for these pollutants and the associated
environmental and health concerns.
2.3.1 Reduced Sulfur Compounds
Emissions of reduced sulfur compounds are associated with the kraft pulping process only. Four
compounds are of concern: hydrogen sulfide (H2S), methyl mercaptan (CH3SH), dimethyl sulfide
[(CH3)2S], and dimethyl disulfide [(CH3)2S2]. These compounds are all derived from sodium sulfide
(NajS), one of the two primary cooking chemicals used in the kraft process, and are generated during the
complex reactions that occur in the initial kraft cook. Major emission sources in the mill include digester
blow and relief gases, evaporator vents, chemical recovery furnace emissions, and pulp washers. They
are also released from vents during a number of other pulping unit processes. Small quantities are
typically dissolved in liquid effluents, which escape from the effluent during wastewater collection and
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Pollution Prevention in Pulp & Paper _____ Section Two
treatment or after discharge to receiving waters. Approximately 12 to 13 kg of total reduced sulfur (or
TRS) are generated per ton of pulp (EPA, 1993).
Even at small concentrations, TRS from kraft pulp mills can be a source of nuisance odors.
Humans can detect the rotten egg and rotten cabbage odors of hydrogen sulfide and methyl mercaptan at
as little as 1 part per billion (ppb). Detection thresholds for dimethyl sulfide and disulfide are about 10
ppb. In untreated kraft mill effluents, TRS may be present in sufficient quantities to taint the taste of fish.
At much higher concentrations these compounds are acutely lethal to marine and terrestrial wildlife
(including humans), but effluent concentrations sufficient to cause acute toxicity are not a concern
associated with well-operated kraft pulp mills. These compounds are not persistent in the environment,
and do not show tendencies to bioaccumulate.
2.3.2 Particulates
The major sources of particulates are fly ash from power boilers, chemical recovery furnaces, and
lime kilns. The highest volume source is the power boiler. Depending on the age and efficiency of the
equipment, the chemical recovery furnace can be a source of very fine particulates, which are a particular
concern because they tend not to settle from the atmosphere and may be associated with more significant
health impacts than larger particulates. The volume of ash generated is a function primarily of the fuel
combusted in the power boiler. Coal and wood produce significant volumes of ash, while oil and other
liquid fuels produce little or no ash.
Particulate emissions from pulp and paper mills are controlled under current EPA regulations (40
CFR Part 60.280), which limit emissions from both power boilers and recovery furnaces.
23.3 Volatile Organic Compounds
Volatile Organic Compounds (VOCs) are organic species that participate in the formation of
photochemical oxidants. Derived from lignin, carbohydrates, and extractives in the pulp furnish, the
largest proportion of these compounds are generated during pulping (and, where used, oxygen
delignification), and the largest emission sources are digester blow gases, the chemical recovery
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Section Two Pollutants of Concern Pollution Prevention in Pulp & Paper
evaporators, and the brownstock washer and knotter hoods (EPA, 1993). Smaller quantities are generated
and emitted during later stages of bleaching and papermaking, as residual VOCs are gradually released
from the pulp. Typical VOC emissions include terpenes, alcohols, phenols, and chloroform (a HAP).
Other VOC species known to be emitted include acetone, methyl compounds, and turpentine-based
organics.
Several VOCs including chloroform, methanol, and other gaseous emissions (including hydrogen
chloride and chlorine) are identified as Hazardous Air Pollutants (HAPs) under the 1990 Clean Air Act
Amendments. Emissions of these compounds from pulp and paper mills will be subject to Maximum
Achievable Control Technology (MACT) emission limitations. These limitations are currently under
development.
2.3.4 Chloroform
Until recently, the pulp and paper industry has been the major industrial source of chloroform
omissions in the United States.13 Chloroform is considered by the EPA, the American Council of
Governmental Industrial Hygienists (ACGIH), and the International Agency for Research on Cancer
(IARC) to be a probable human carcinogen, posing a significant cancer risk at an exposure level of 50
parts per million. In 1989, the Occupational Safety and Health Administration (OSHA) proposed revising
its permissible exposure limit (PEL) for chloroform from 50 ppm to 2 ppm (54 FR 12, January 19, 1989).
Chloroform is among the emissions that are being addressed under two ongoing EPA programs: The
voluntary Industrial Toxics Program sponsored by the Administrator's office, and the MACT standard
described above.
Chloroform generation is associated with chlorine-based bleaching of pulps. The largest
chloroform source is the sodium hypochlorite (NaOCl) bleaching stage (where this stage is used), although
13 The identification of the hypochlorite bleaching stage as a major source of chloroform, and its
subsequent steady elimination from bleach sequences, has led to significant decreases in chloroform
emissions by the industry.
Page 2-19
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Pollution Prevention in Pulp & Paper Section Two
elemental chlorine and chlorine dioxide stages are also responsible for some chloroform generation. The
rate of chloroform generation is a function of many variables, including:14
Hypochlorite charge - Chloroform generation increases with the charge of the hypochlorite used
in bleaching, at a rate proportional to the square root of the amount of hypochlorite used;
Lignin content - Chloroform generation in the hypochlorite bleaching state is proportional to the
lignin content of the bleached and extracted pulp that enters this stage (expressed at the chlorine
extracted kappa number, or CEK);
Chlorine factor The chlorine factor expresses the ratio of molecular chlorine to pulp.
Chloroform emissions increase with increasing chlorine factor;
pH Emissions tend to increase with increasing pH during bleaching and extraction. Acid sewer
effluents containing chloroform precursors are often mixed with more alkaline waste streams, and
the resulting pH increase is associated with significant chloroform formation;
Chlorine dioxide substitution Increasing chlorine dioxide substitution generally results in a
reduction in chloroform formation; and
Chlorination stage temperature - A reduction in chlorination stage temperature tends to reduce
chloroform emissions.
Chloroform discharges are divided between effluents and emissions to air. Sixty percent or more
of total discharges are typically in the form of fugitive air emissions from bleach plant vents. The
majority of chloroform that remains in bleach plant effluents is also ultimately released to the atmosphere
in the form of evaporative emissions from the wastewater treatment system or from the receiving waters.
2.3.5 Other Hazardous Air Pollutants (HAPs)
In addition to chloroform, the kraft pulping and bleaching processes emit quantities of other HAPs
including methanol, hydrogen chloride, and chlorine. Methanol is the largest volume HAP, and is emitted
from several sources. The black liquor oxidation stage, where used, is the largest volume methanol source
14 Sources for all factors cited are NCASI: Dallons and Crawford (1990); Dallons et al. (1990);
Crawford et al., (1991).
Page 2-20
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Section Two Pollutants of Concern Pollution Prevention in Pulp & Paper
according to most recent estimates.15 Newer recovery boiler designs eliminate the black liquor oxidation
stage in favor of indirect contact evaporation, thus newer mills will emit substantially lower amounts of
methanol. Much smaller amounts of methanol are emitted from digester blow valves, knotter and washer
hood vents, evaporator vents, and turpentine recovery processes, as well as from acid sewers at plants
practicing high chlorine dioxide substitution.
Hydrogen chloride is emitted from several sources including washer and seal tank vents (especially
under high or 100 percent C1O2 substitution), while chlorine is released from C-, D-, and H-stage tower
and washer vents (EPA, 1993).
15 The most recent document characterizing air emissions from pulp and paper facilities (EPA, 1993)
covers only the evaporation portion of the recovery process and thus does not address emissions from
black liquor oxidation. Additional recovery processes will be discussed in future drafts of this document.
Page 2-21
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Pollution Prevention in Pulp & Paper Section wo
SECTION TWO REFERENCES
Berry et al., 1989. R.M. Berry, B.I. Fleming, R.H. Voss, C.E. Luthe, and RE. Wrist, 'Toward Preventing
the Formation of Dioxins During Chemical Pulp Bleaching," Pulp & Paper Canada, September,
1990, p. 48.
Brunsvik et al., 1991. 'To CD or Not to CD. That is the Question," Proceedings, 1991 TAPPI Pulping
Conference., p. 159.
Crawford et al., 1991. Robert J. Crawford, Victor J. Dallons, Ashok K. Jain, and Steven W. Jett.
"Chloroform Generation at Bleach Plants With High Chlorine Dioxide Substitution and/or Oxygen
Delignification," Proceedings, 1991 TAPPI Environmental Conference, p. 305.
Dallons et al., 1990. Victor J. Dallons, Dean R. Hoy, Ronald A. Messmer, Robert J. Crawford,
"Chloroform Formation and Release From Pulp Bleaching," TAPPI Journal, June 1990, p. 91.
EPA, 1988. U.S. Environmental Protection Agency. U.S. EPA/Paper Industry Cooperative Dioxin
Screening Study, Office of Water Regulations and Standards, Washington, D.C. March 1988.
EPA 440/1-88-025.
EPA, 1993. U.S. Environmental Protection Agency. Pulp, Paper, and Paperboard Industry - Background
Information for Proposed Air Emission Standards. Office of Air Quality Planning and Standards,
Research Triangle Park, NC. Preliminary Draft. April 1993.
Folke et al., 1991. Jens Folke, Karl Johan Lehtinen, Howard Edde. "The Scientific Foundation of
Adsorbable Organochlorines (AOX) as a Regulatory Parameter for Control of Organochlorine
Compounds," Proceedings 7997 TAPPI Environmental Conference, p. 517.
Geil, 1993. Personal communication between Jeff Cantin of ERG and Steve Geil, EPA Office of Water,
Office of Wastewater Enforcement and Compliance, Permits Division (Washington, D.C.).
February 18, 1993. Based on data pulled from the Office of Water's Permit Compliance System.
Germgard, 1988. Ulf Germgard. "Chlorate Discharges From Bleach Plants - How To Handle a Potential
Environmental Problem," Proceedings, 1988 TAPPI Pulping Conference p. 315.
Germgard, 1983. Ulf Germgard. "Oxygen Bleaching and its Impact on Some Environmental Parameters,"
Svensk Paperstiding 88(12).
Ho et al., 1991. Bosco P. Ho, Randy R. Warner, Denis E. Hassick, Terrance J. McLaughlin, Charles
Ackel. "Automatic Coagulant Dosage Control for Mill Wastewater Color Removal," Proceedings
7997 TAPPI Environmental Conference, p. 617.
Lehtinen et al., 1991. K.-J. Lehtinen, B. Axelsson, K. Kringstad, L. Strombers. "Characterization of Pulp
Mill Effluents by the Model Ecosystem Technique. SSVL Investigations in the Period 1982-
1990," Nordic Pulp and Paper Research Journal. 6(2): 81-88.
Page 2-22
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Section Two Pollutants of Concern Pollution Prevention in Pulp & Paper
SECTION TWO REFERENCES (cont)
MFC, 1991. Internal Reports of the European Environmental Research Group. Denmark, Sweden, and
Finland.
MacKay, 1989. Donald MacKay. "A Review of the Nature and Properties of Chemicals Present in Pulp
Mill Effluents," Chemosphere, No. 7 1988, p. 248.
Miner and Unwin, 1991. Reid Miner and lay Unwin. "Progress in Reducing Water Use and Wastewater
Loads in the U.S. Paper Industry," TAPPI Journal, August, 1991, p. 127.
Ontario Ministry of the Environment, 1988. Kraft Mill Effluents in Ontario. Municipal-Industrial
Strategy for Abatement. March 1988.
OTA, 1989. U.S. Congress, Office of Technology Assessment. Technologies for Reducing Dioxin in the
Manufacture of Bleached Wood Pulp, OTA-BP-O-54 (Washington, D.C.: U.S. Government
Printing Office, May 1989).
Reeve and Earl, 1989. Douglas W. Reeve and Paul F. Earl. "Chlorinated Organic Matter in Bleached
Chemical Pulp Production: Part I Environmental Impact and Regulation of Effluents," Pulp &
Paper Canada, April 1989, p. 65.
Rush and Shannon, 1976. Review of Color Removal Technology in the Pulp and Paper Industry.
Environment Canada, Water Pollution Control Directorate, Report EPS #WP 765.
Springer, 1986. Aan M. Springer. Industrial Environmental Control Pulp and Paper Industry, lohn
Wiley & Son, New York.
Van der Krakk et al., 1992. "Receiving Water Environmental Effects Associated With Discharges from
Ontario Pulp Mills," Proceedings, 19th Annual Toxicity Workshop, Edmonton, Alberta, Canada,
Oct. 4-7, 1992.
Page 2-23
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Section Three Woodyard Operations Pollution Prevention in Pulp & Paper
SECTION THREE
POLLUTION PREVENTION TECHNOLOGIES IN WOODYARD
AND CHIPPING OPERATIONS
This section reviews the operations in the pulp mill woodyard and chipping areas and identifies
technologies relevant to prevention of pollution associated with these activities. The main
technologies discussed are raw material selection (Section 3.1), recycle of log flume water
(Section 3.2), dry debarking (Section 3.3), unproved pulp chipping and screening (Section 3.4),
and stormwater management (Section 3.5).
3.1 RAW MATERIAL SELECTION
Increasingly, pulp mills have turned to using sawmill residues (logs, chips, sawdust) to supplement
the virgin fiber used in kraft pulping and papermaking. While use of sawmill residues improves the
industry's overall product yield from timber, problems may arise at the pulp mill if the raw material has
been previously treated with wood preservatives, specifically pentachlorophenols (PCPs). Researchers at
PAPRICAN (Pulp and Paper Research Institute of Canada) have found that these types of preservatives
contain CDDs/CDFs or CDD/CDF precursors, which can carry over into the bleach plant (Luthe et al.,
1992.; Berry et al., 1989; Voss et al., 1988). There, they may react with chlorine to form dioxins and
furans.
While the use of chlorophenol-based preservatives has been more common in Canada (and
particularly in British Columbia), raw material supplies for U.S. mills may include wood from a variety
of sources. Mills should therefore try to avoid using chips, logs, or sawdust from unknown sources
without first testing for the presence of chlorophenols.
The costs of using "cleaner" raw materials may be slightly higher if lower prices are paid for the
treated wood or wood waste. It is widely believed, however, that contaminated wood fiber is not a major
problem hi the U.S. pulp and paper industry.
Page 3-1
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Pollution Prevention in Pulp & Paper Section Three Woodyard Operations
3.2 RECYCLE OF LOG FLUME WATER
Log flumes are used at some mills to transport wood from log piles to debarkers and chippers.
The water used to convey the logs can be recycled, with fiber and bark being recovered and burned in a
furnace (the "hogged fuel" boiler) for heat recovery. Alternatively, or in addition, treated wastewater can
be used as makeup for the log flume.
The practice of log flume water recycle is common among mills that use log flumes. Costs of
developing an appropriate recycle system may be in the range of $100,000 to $500,000. Recycle of log
flume water will reduce the discharge of BOD5 and TSS, as well as conserve water. BOD5 and TSS
reductions of up to 750 Ibs per day have been previously estimated for a 3 MGD recycle system (U.S.
EPA, 1982).
3.3 DRY DEBARKING
The bark of the tree comprises about 10 percent of the weight of the tree trunk. Bark does not
yield good papermaking material because it is resistant to pulping, contains a high percentage of
extractives, and retains dirt. In most pulping processes, bark is removed from the logs before they
undergo chipping.
The most common debarking mechanism used for pulpwood is the debarking drum, which
removes bark by tumbling the logs together in a large cylinder. Slots in the outside of the drum allow
the removed bark to fall through. The bark collected from these operations is usually fed to the hogged
fuel boiler and used to generate process heat or steam.
Wet drum debarkers rotate the logs in a pool of water and remove bark by knocking the log
against the side of the drum, while hydraulic barkers employ high pressure jets of water to remove the
bark. The water used for bark removal in these systems is typically recycled, but a certain amount is lost
as overflow to carry away the removed bark. This overflow can contain resin acids and highly colored
materials which leach out of the bark and into the waste water stream. This effluent stream is collected
and routed to the wastewater treatment system, where the pollutants are normally removed quite effectively
by the biological processes that take place there (Springer, 1986).
Page 3-2
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Section Three Woodyard Operations Pollution Prevention in Pulp & Paper
Dry debarking methods such as dry drum debarkers (see Figure 3-1) or shearbarkers eliminate the
water stream and the pollutants associated with it. Dry debarkers already dominate the industry, and wet
systems have been in the process of being phased out since the 1970s (Smook, 1982).
The costs of dry drum debarkers should not differ significantly from a wet system. Costs for
replacing both types of equipment are in the range of $10 to 20 million.
3.4 IMPROVED CHIPPING AND SCREENING
The purpose of chipping is to reduce the logs to a smaller size suitable for pulping. In the
conventional chipper, logs are fed into a chute where they contact a disc outfitted with a series of radially-
mounted blades. The blades project about 20 mm from the disc. Chip uniformity is extremely important
for proper circulation and penetration of the pulping chemicals, hence considerable attention is paid to
operational control and maintenance of the chipper. Chips between 10 and 30 mm in length, and 2 to 5
mm in thickness, are generally considered acceptable for pulping.
The chipped wood is passed over a vibrating screen that removes undersized particles (fines) and
routes oversized chips for rechipping. Normally, fines are burned with bark as hogged fuel, although they
may also be pulped separately in specialized "sawdust" digesters. In most mills, chips are segregated only
according to chip length.
Chip thickness screening has become important as mills realize the need to extend delignification
and reduce bleach plant chemical demands. Both absolute chip thickness and thickness uniformity have
a significant impact on delignification, since the kraft cooking liquor can only penetrate the chip to a
certain thickness (Tikka et al., 1992). Thin chips are easier to cook to lower kappa numbers. Uncooked
cores from over-thick chips will lower the average kappa reduction of a cook and contribute to higher
bleaching chemical demands. To improve thickness uniformity, many mills are now adopting screening
equipment that separates chips according to thickness (Strakes and Bielgus, 1992). Chips that exceed the
maximum acceptable thickness are diverted to a chip sheer that cuts them radially and reintroduces them
to the screening system (see Figure 3-2). Costs for chip thickness screening and reprocessing of between
$2 and $4 million have been cited for new installations (U.S. EPA, 1992). Costs for retrofits would
generally be higher.
Page 3-3
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Pollution Prevention in Pulp & Paper
Section Three - Woodyard Operations
Figure 3-1. Debarking drum.
Source: Beak, 1978.
Page 3-4
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Section Three Woodyard Operations
Pollution Prevention in Pulp & Paper
DRUM SEGMENT
GAGE PLATE
KNIFE
KNIFE CLAMP-
Figuie 3-2. Chip thickness slicer showing oversize chip being split.
Source: Smook, 1982.
Page 3-5
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Pollution Prevention in Pulp & Paper Section Three Woodyard Operations
3.5 STORMWATER MANAGEMENT
The impact of storm water runoff from industrial facilities can be significant and has begun to be
addressed by regulatory actions.1 At the pulp mill, the runoff from wood and chip storage and processing
areas is of greatest concern, as these streams may contribute substantially to BOD5 and TSS loadings.
Options for reducing stormwater impacts on receiving waters include modifying wood yard
operations to reduce storm run-off (i.e., moving operations inside, where feasible), and installing curbing,
diking, and drainage collection for storm water from chip piles and wood processing areas. Storage and
treatment of collected stormwater may be required. Collected stormwater can be transported to the
wastewater treatment facility, which should effectively remove the pollutants of concern.
Costs for stormwater collection and treatment are variable and quite site-specific. They depend
more on the current configuration of the mill woodyard and location of treatment facility than on the
particular type of controls installed.
1 Regulations for permitting stormwater discharges from industrial facilities were promulgated in
November, 1990 (55 FR 47990).
Page 3-6
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Section Three Woodyard Operations Pollution Prevention in Pulp & Paper
SECTION THREE REFERENCES
Berry et al., 1989. R.M. Berry, L.H. Allen, B. I. Fleming, R.H. Voss, C.E. Luthe, P.E. Wrist. 'Toward
Preventing the Formation of Dioxins During Chemical Pulp Bleaching," Pulp and Paper Canada
90(8), 1989, pp. 48-58.
Edde, 1984. Howard Edde. Environmental Control for Pulp and Paper Mills. Noyes Publications Park
Ridge NJ.
Luthe et al., 1992. C.E. Luthe, R.M. Berry, R.H.Voss. "Chlorinated Dioxins in the Production of
Bleached Kraft Pulp Resulting from the Use of Sawmill Wood Chips Contaminated with
Polychlorinated Phenols," Proceedings, 7992 TAPPI Environmental Conference, Richmond VA
April 1992. p. 859.
Springer, 1986. Allan M. Springer. Industrial Environmental Control Pulp and Paper Industry, John
Wiley & Sons, New York.
Strakes and Bielgus, 1992. George Strakes and Joe Bielgus. "New Chip Thickness Screening System
Boosts Efficiency, Extends Wear Life," Pulp and Paper, July 1992, p. 93.
Tikka et al., 1992. P.O. Tikka, H. Tahkenen, K.K. Korasin. "Chip Thickness vs. Kraft Pulping
Performance, Part I: Experiments by Multiple Hanging Baskets," Proceedings, 7992 TAPPI
Pulping Conference, Boston MA, November 1992, p. 555.
U.S. EPA, 1982. Development Document for Effluent Limitations Guidelines and Standards for the Pulp,
Paper and Paperboard. Effluents Guidelines Division, WH-552, Washington, D.C. EPA 440/1-
82/025, October, 1982.
U.S. EPA, 1992. Model Pollution Prevention Plan for the Kraft Segment of the Pulp and Paper Industry.
Region 10, Seattle, WA. EPA 910/9-92-030, September, 1992.
Voss et al., 1988. R.H. Voss, C.E. Luthe, B.I. Fleming, B.H. Allen. "Some New Insights into the
Origins of Dioxins Formed During Chemical Pulp Bleaching," in Proceedings, 1988 CPPA
Conference, Vancouver, B.C., October 25-26, 1988.
Page 3-7
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Section Four - Pulping Pollution Prevention in Pulp & Paper
SECTION FOUR
POLLUTION PREVENTION TECHNOLOGIES IN PULPING OPERATIONS
This section discusses the conventional pulping processes and identifies pollution prevention
technologies that can be implemented in the pulp mill to effect environmental improvements.
This section (and the rest of the report) retains the traditional distinction between pulping and
bleaching, which is to classify stages that occur prior to the application of chlorine-based
bleaching agents as pulping stages, and those that occur following application of these agents as
bleaching stages. With the increased use of non chlorine-based delignification agents, however,
these distinctions are becoming less and less meaningful. For optimum pollution prevention
potential, many experts would recommend viewing pulping and bleaching as integrated processes.
4.0 INTRODUCTION
The purpose of pulping is to separate the tightly-bound fibers in the wood chips into individual
fibers so they can be formed into a sheet on the papennaking machine. The type of pulping process used
depends first on the desired properties of the end product, and second on the relative economics of raw
material costs, energy requirements, and, increasingly, effluent and emissions treatment and control costs.
Chemical pulping methods use various chemical solutions to dissolve the lignin that holds the
wood fibers together, while mechanical methods use mechanical energy (e.g., grinding) to tear the fibers
from the wood. Chemical pulping differs from mechanical methods in that lignin and other materials are
dissolved and removed during processing. These losses are reflected in a lower yield (40 to 55 percent),
in comparison with mechanical pulping methods (up to 95 percent yield). While the lower yield means
that larger quantities of wood are required to produce the same quantity of paper, lignin removal enables
the mill to produce a pulp with superior papennaking characteristics. Figure 4-1 shows the process flow
for a typical chemical (kraft) pulp mill.
The amount of lignin remaining in the fiber following chemical pulping is an important pulping
control parameter. The most common method for measuring lignin content is the kappa number test. The
kappa number of a pulp is based on the amount of potassium permanganate required to oxidize the lignin
Page 4-1
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"d
Ł
a
WOOD
PREPARATION
PULPING
BLEACHING
STRIPPED
CONDEN8ATEB
I
Figure 4-1. Process Flowsheet for Conventional Integrated Bleached Kraft Mill (Mid- to Late-1980s).
'MAIN PULP UNE
^BLACK LIQUOR
> OTHER FLOWS
Source: Eastern Research Group, Inc.
-------
Section Four - Pulping Pollution Prevention in Pulp & Paper
contained in a standard sample of dry pulp. Although this is a strictly chemical test, pulp chemists have
devised a number of rapid scanning techniques to provide on-line measurement of kappa number during
pulping and bleaching.
Monitoring of the pulp kappa number provides feedback to the mill operators concerning how far
along the pulping and bleaching reactions have proceeded. Lower kappa numbers correspond to a lower
residual lignin concentration. A pulp with a kappa number of 35 has a lignin content of approximately
5 percent. Most pulp is cooked to a target kappa number, measured at the point in the process where pulp
exits the brownstock washer following cooking and before proceeding to any bleaching stages (see Figure
4-1).1 Kappa number targets are usually between 20 and 40 for softwood pulp and between 15 and 25
for hardwood.
The kappa number of brownstock pulp determines its bleachability in subsequent bleaching stages.
The term "bleachability" refers to the amount and also the type of bleaching chemicals that will be
required to produce pulp of the desired brightness. The more lignin that is removed in the pulping stages,
the lower the downstream bleaching chemical demand. As brownstock kappa numbers fall, it also
becomes possible to bring the pulp to target brightness levels using environmentally preferable bleaching
chemicals (e.g., less chlorine and more chlorine dioxide, oxygen, or peroxide). Lowering the brownstock
kappa number to reduce bleaching chemical demands is a major objective of the pollution prevention
technologies discussed in this section.
Using conventional pulping methods, the brownstock kappa can be reduced below the target levels
cited above, e.g. through longer residence times in the digester or more severe cooking conditions. While
this will increase its bleachability, it also results in a loss of pulp yield and strength, as the pulping
chemicals become less selective and begin to attack the cellulose material.2 As discussed in this section,
however, new techniques (generally known as extended cooking) have been devised that enable the lignin
content to be reduced without further harming the pulp fibers.
1 The term brownstock refers to the raw pulp exiting the pulping stage. At this point, the pulp still
contains large amounts of reacted lignin and spent cooking chemicals. The brownstock pulp moves
through a series of washing and screening/cleaning stages before it enters the bleach plant.
2 The term selectivity refers to the differential rates of reaction between the pulping chemicals and
lignin, and the pulping chemicals and cellulose.
Page 4-3
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Pollution Prevention in Pulp & Paper Section Four - Pulping
4.1 CONVENTIONAL KRAFT PULPING
Kraft (or sulfate) pulping has become the most common chemical pulping process in use in the
United States, accounting for approximately 77 percent of pulpmaking capacity in 1990 (API, 1992). The
success and widespread adoption of the kraft process is due to several factors. First, because the kraft
cooking chemicals are selective in their attack on wood constituents, the pulps produced are notably
stronger than those from other processes. The kraft process is also flexible, in so far as it is amenable
to many different types of raw materials and can tolerate contaminants that may be found in the wood
(e.g., high resin content). Finally, the chemicals used in kraft pulping are readily recovered within the
process (see below).
Kraft processing subjects the wood chips to a mixture of caustic soda (sodium hydroxide, or
NaOH) and sodium sulfide (NajS) in a reaction vessel known as a digester. This chemical mixture is
known as white liquor (or cooking liquor). The caustic hi the liquor attacks the lignin hi the fiber,
breaking it into smaller segments that are soluble in the cooking liquor. The output of kraft pulping
consists of the separated wood fibers (brownstock pulp) and a black liquor that contains the dissolved
lignin solids in a solution of reacted and partially reacted pulping chemicals.
Kraft pulping techniques are available for both batch and continuous operation. These are
discussed briefly in rum below.
4.1.1 Batch Pulping
In batch operation, the digester is charged with chips and the cooking liquor, a mix of sodium
hydroxide (NaOH) and sodium sulfide (NajS). Figure 4-2 shows a diagram of a batch digester. The
liquor is circulated through the digester, and the temperature and pressure are raised to the level required
for reaction. Once the reaction is complete, the pulp is emptied by opening the "blow" valve at the
bottom of the digester. Pressure built up inside the digester during reaction is sufficient to push most of
the pulp out into the blow tank. Steam or waste cooking liquor may be used to flush the remaining pulp
from the digester. As the pressure is relieved and the temperature of the cooking mixture drops, steam
and gases formed during the reaction are released. These gases and liquids must be captured and
Page 4-4
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
C«t OlflUttT Cot>dAngliV
-------
Pollution Prevention in Pulp & Paper Section Four - Pulping
controlled, as they are a source of concentrated odor (TRS), volatile organic compounds (VOC), and
biochemical oxygen demand (BOD5) pollutants.
Although the digester operation is performed in batches, the downstream bleaching is performed
continuously. To supply a continuous feed of pulp to the bleaching operation, a series of batch digesters
are usually operated on a staggered basis. Individual digesters sometimes range in size from 6,000 to
8,000 cubic feet.
Following removal from the digester, the pulp may pass through a disc refiner to separate the
fibers. It is then screened to remove knots and unreacted chips, or "rejects". These may be either sent
through the digester again or used as fuel in the hog fuel boiler.
The screened pulp then passes through a series of washers. Washing is a key stage, since
dissolved lignin that cannot be removed here will continue on to the chlorination stage, where it consumes
a large amount of bleaching chemicals, reacts with the bleaching chemicals to form chlorinated organics,
and will be discharged with the bleach plant effluent. The mixture of dissolved lignin and cooking liquor
washed from the pulp is known as black liquor. The washing systems used to remove this spent cooking
solution from the pulp are quite sophisticated. Most mill wash systems operate in a countercurrent
fashion, with water flowing in a direction opposite to the pulp. The pulp is alternately thickened and
diluted on the washing drums, with water sprayed onto the pulp mat to displace the suspended black liquor
solids. As shown in Figure 4-3, the filtrate from each stage is used as washing liquid in the previous
stage. This configuration conserves water and removes chemicals from the first washing stage most
effectively.3
Final thickening of the pulp is performed using a gravity thickener or "decker". From the decker,
the pulp may be blended with other pulps before it either continues on to the bleach plant or is dried for
shipping to market.
3 Note that the figure illustrates countercurrent washing in conventional drum washers. More recently,
the industry has shifted towards more advanced washing system designs (see Section 4.8). Although the
washing equipment differs, the same principle of countercurrent wash water flow is generally applied.
Page 4-6
-------
Brown-
stock V > '
Counterflow water for washing & dilution
Fresh water
Concentrated
black liquor
Weak liquor
Weak liquor
Figure 4-3. Countercurrent brownstock washing, using drum washers.
Source: Kline, 1982.
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Pollution Prevention in Pulp & Paper Section Four - Pulping
4.1.2 Continuous Pulping
In Figure 4-4, the continuous cooking process is diagrammed. The most common type of
continuous digester is the Kamyr downflow digester, introduced by Kamyr in 1950. Chips are first fed
from a chip meter into the steaming vessel. The chips are carried through a series of tubes and are
injected with cooking liquor. The treated chips are then deposited into the top of the continuous reaction
vessel. The chip mass travels downward through the digester via gravity, during which time the liquor
penetrates the chips and the lignin is dissolved.
In continuous pulping, washing of the pulp takes place hi a section of the digester known as the
washing zone. Here, the spent liquor is siphoned off and hot water is introduced for washing. The wash
water is circulated through a heat exchanger that gradually reduces the temperature of the chips, thereby
avoiding the flashing of volatile gases that occurs during batch operation. This washing method, known
as diffusion washing, also operates in a countercurrent fashion. It is capable of removing about 98 percent
of the black liquor solids. To complete the washing, the pulp is diluted to approximately 2 percent
consistency4 and is then washed over a series of rotary drum washers. As hi batch pulping, a decker is
normally used for final thickening of the washed pulp.
4.1.3 The Kraft Recovery Cycle
In addition to producing pulp with superior strength properties, a primary advantage of the kraft
process is the relative ease of recovery of the pulping chemicals. The recovery side of the mill's
operations consists of various stages that serve to concentrate, purify, and reconstitute the kraft cooking
chemicals from the black liquor, while recovering energy value from the dissolved lignin and other solids
removed during pulping and washing.
The bottom half of Figure 4-1 shows the rather complex chemical recovery system that is
characteristic of all kraft mills. Chemical recovery begins with the spent cooking chemicals and
solubilized lignin that is flushed from the cooked pulp at the brownstock washers. This mixture, known
as weak black liquor, is concentrated in a series of multiple effect evaporators to form strong black liquor,
4 Pulp consistency is usually expressed as the ratio of the oven-dry weight of pulp solids to the total
weight of the pulp slurry.
Page 4-8
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
LOW-PRESSURE STEAM
TO CONDENSES
CHIPS
UPPER SPARE LOWER
HEATER HEATER HEATER
TO RASH HEAT EVAPORATORS
TO STORAGE
Figure 4-4. Continuous digester and ancillary equipment.
Source: Smook, 1982.
Page 4-9
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Pollution Prevention in Pulp & Paper Section Four - Pulping
with a solids content of between 60 and 80 percent. The strong black liquor is fired into the recovery
boiler, where the heat content of the organic lignin solids is released to generate steam for process use.
Most mills are able to generate 100 percent of their energy requirements in this manner.
Smelt from the bottom of the furnace (consisting of sodium/sulfur salts and inorganic chemicals)
runs into a dissolving tank where it is mixed with weak wash, the filtrate from lime mud washing (see
below) to form green liquor. The green liquor is clarified to remove carbonaceous ash residues and other
impurities. These dregs, as they are known, are washed to remove soluble sodium salts while the
remaining residue is removed and generally disposed of in a landfill. The clarified green liquor moves
next to the calcining system, where it is mixed with calcium hydroxide in the slaker to convert sodium
carbonate to sodium hydroxide (caustic), one of the principal pulping chemicals.
Calcium carbonate precipitates out of the causticizers and is removed in the white liquor clarifier.
The clarified solution then contains the two major cooking chemicals, sodium hydroxide (NaOH) and
sodium sulfide (Na^). This white liquor is then ready for reuse in the pulp digester(s).
The final stage in the recovery process involves recycle of the precipitated calcium carbonate
removed from the white liquor clarifier. This mixture is thickened, washed, and introduced into the lime
kiln, which converts it to calcium oxide. The calcium oxide is recovered and used in the slaker, as
described above.
4.2 EXTENDED DELIGNIFICATION
The amount of bleaching power (or bleaching chemicals) required in the bleach plant to bring the
pulp to the target brightness level is directly related to the kappa number (residual lignin content) of the
brownstock pulp, i.e. the amount of lignin that remains in the pulp following chemical cooking. The mill
can reduce the bleaching chemical demands (and subsequent environmental effects) by adopting techniques
that reduce the brownstock kappa number. Over the last decade, methods and equipment have been
developed that allow the pulp cooking time to be extended, enabling further delignification to occur before
the pulp moves on to the bleach plant. At the same time, extended cooking, as it is known, protects the
pulp from the detrimental effects (reduced quality and yield) that would normally accompany increased
cooking time.
Page 4-10
-------
Section Four - Pulping Pollution Prevention in Pulp & Paper
How It Works
In conventional kraft pulping, the digester is filled with chips and then given a one-time charge
of cooking chemicals (sodium hydroxide and sodium sulfide). The alkali concentration in the reactor is
initially high, but then falls as the cook progresses and the cooking chemicals are consumed. Normal
reaction times are between one and three hours. Longer cooks will farther reduce lignin content but will
also begin to degrade the cellulose, as the reactions become less selective towards lignin
The ability to extend the cooking process without impacting pulp quality has been achieved by
applying principles developed by the Swedish Forest Products Research Institute (STFI) in the late 1970s
(Hartler, 1978). The technique involves charging the cooking chemicals at several points throughout the
cook. This levels out the alkali profile in the pulp, permitting more lignin to be dissolved in the latter
stages of the process. The specific techniques used to achieve this effect are proprietary, however their
general operation is well-known and have been widely-adopted. Improved selectivity is obtained through
careful application of the following principles:
(1) Achieve a more uniform concentration of effective alkali (NaOH + Vi NajS) throughout
the cook (lower at beginning, higher at end);
(2) Maximize the concentration of hydrogen sulfide ions (HS~0, especially during the initial
phase of the cook;
(3) Minimize the concentration of dissolved lignin at the end of the cooking process; and
(4) Maintain low temperatures at the beginning and end of the cook.
By attaining greater control over these conditions in the digester, the delignification reaction can
be extended, and the lignin content of the brownstock pulp reduced by between 20 and 50 percent
compared to conventional digesters. In the absence of such controls, similar lignin reductions could not
be accomplished without significant losses in pulp yield and strength.
Page 4-11
-------
Pollution Prevention in Pulp & Paper Section Four - Pulping
The Modified Continuous Cook (MCC®) and Extended Modified Continuous Cook (EMCC®)
processes were developed by Kamyr for continuous pulping operations.5 MCC® has been described as
"just in time chemistry" — the system is optimized to deliver the right amount of the right kind of
chemicals at the right time. In MCC® cooking, the kraft digester is modified so that liquor is introduced
at several different points to maintain a constant alkali concentration throughout the cook (see Figure 4-5).
In EMCC®, about 20 to 25 percent of the white liquor is added to the wash liquor in the bottom zone of
the digester for counter-current cooking (see Figure 4-6). This levels out the alkali profile through the
cook, enabling further delignification to occur. EMCC® can be implemented without MCC® and in faci
is the normal way of converting a conventional continuous digester to extended cooking.
By splitting the addition of cooking liquor and improving liquor circulation and mixing, modified
cooking processes level out the alkali concentration not only from the beginning to the end of the cook
but also throughout the length of the digester. The more uniform cooking of chips throughout the digester
helps maintain pulp yield, (fewer under- and over-cooked chips), and leads to easier bleachability and
reduced bleaching chemical demand.
For batch pulping, the cook can be extended using the Rapid Displacement Heating (RDH) process
or one of its variations. RDH was originally developed by the Beloit Corp. (Beloit, Wisconsin).
Adaptations of the RDH principles are available in the SuperBatch™ technology of Sunds Defibrator
(Sundsvall, Sweden; Norcross, Georgia) and the Enerbatch® process of Voest-Alpine (Linz, Austria). The
basic liquor displacement cycle for RDH pulping is illustrated in Figure 4-7 and includes five stages:
(1) Chips are fed and steam packed into the digester;
(2) Warm black liquor at about 115 °C is used to impregnate the chips, improving penetration
of the cooking chemicals;
(3) The warm liquor is displaced with hot black and hot white liquor (150°C to 155°C), which
is raised to cooking temperature using indirect heating;
(4) After time at temperature, the hot cooking liquor is displaced using brownstock washer
filtrate, which is cooled using indirect cooling to below flashing temperature (95°C). The
displaced hot liquor is stored and used in subsequent cooks; and
5 Both Kamyr AB (Karlstad, Sweden) and Kamyr, Inc. (Glens Falls, New York) were once part of
the same company. Both now compete in the North American market subject to certain legal restrictions
currently being litigated. Kamyr AB is owned by Kvaerner of Norway and Kamyr Inc. is owned by
Ahlstrom of Finland.
Page 4-12
-------
Single Vessel Hydraulic Digester
Top Separator
Hsittfl
FI.Wi
Tank
No, I
Oullel 0«>!ci
Single Vessel Hydraulic Digester, MCC Adaptation
Toe Sioatitor
Ouilft
Figure 4-5. Equipment diagram for MCC extended cooking, showing multiple liquor addition points.
Ł
LO
Source: Kamyr, Inc.
-------
era
CD
4^
Ł
Single Vessel Hydraulic Digester
Top Sepintor
Flisti
Til*
No. I
V
^r
l Dtvrct
Single Vessel Hydraulic Digester, EMCC Adaptation
WHTEUQUOfl
Oullel Oevict
Figure 4-6. Equipment diagram for EMCC extended cooking, showing multiple liquor addition points and liquor addition to wash zone.
Source: Kamyr, Inc.
-------
Section Four - Pulping
Pollution Prevention in Pulp & Paper
PUMPED DISCHARGE
OR AIR BLOW
DISPLACEMENT
CHIPS
~*
CHIP FILL
LIQUOR
'DISPLACEMENT
TECHNOLOGY
FOR BATCH
DIGESTERS
ru
DISPLACEMENT
TIME AT TIME TO
TEMPERATURE TEMPERATURE
WARM BLACK
LIQUOR FILL
HOT BLACK
LIQUOR FILL
HOT WHITE
LIQUOR FILL
Figure 4-7. The Rapid Displacement Heating (RDH) cycle for batch digester systems.
Source: Macleod, 1992.
Page 4-15
-------
Pollution Prevention* Pulp & Paper Section Four - Pulping
(5) The reacted pulp is pumped or air blown out of the digester vessel.
The chemical delivery demands of an RDH-type batch system require additional equipment and
a sophisticated process control system. This is especially true where the mill may run as many as 20
batch digesters at a time, and where common process equipment (accumulator tanks, pumps, piping) may
be shared. Production scheduling can be thrown off if there are disruptions due to equipment failure or
operator error. Much of the work done since the discovery of the RDH principles has involved refinement
of the "tank farm" configuration and improvement to the distributed control system that oversees the
process.
4.2.1 Number of Installations
Macleod (1992) has recently presented a survey of extended cooking installations worldwide.
These and additional listings compiled by other industry observers are shown in Table 4-1. At the time
of this writing, world capacity for extended cooking was about 11 million tons per year, representing 20
percent of bleached kraft capacity.
Worldwide, 31 new MCC®/EMCC® systems and 15 retrofits have been installed or are currently
underway. Twenty-five of these projects are in the United States, including 16 new installations and 9
retrofits. Combined U.S. MCC® and EMCC® capacity (installed and underway) is 35,255 tons per day
(tpd), representing about 25 percent of U.S. bleached chemical pulp capacity. The average capacity of
these installations is over 1,100 tpd. Stromberg (1993) reports that all new Kamyr digesters sold since
1985 have been equipped with, or prepared for, MCC® operation, hence retrofitting newer continuous
digesters should be a relatively straightforward process.
Installations of extended batch cooking are also shown in Table 4-1. As of this writing, Beloit's
RDH system had been installed at four U.S. mills plus one each in Canada, Finland, Spain and Taiwan.
All four U.S. installations were new installs as opposed to retrofits. The Sunds SuperBatch™ digester
system is currently operating at the Jefferson Smurfit mill in Jacksonville, Florida, at three Scandinavian
mills, and at one South African mill.
Page 4-16
-------
Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-1
Installations of Extended Delignification Systems Worldwide
Company
Location
Fiber
Furnish
Capacity
(tons/day)
Start-
Up
New Kamyr MCC®/EMCC® Continuous Digesters
North American Mills
Federal Paperboard
Domtar
Longview Fibre
Howe Sound
Weyerhaeuser
Daishowa Canada
Weldwood of Canada
Federal Paperboard
Champion
International
Union Camp
Stone Savannah River
Alabama Pine Pulp
Gulf States Paper
Temple-Inland
Union Camp
Alberta-Pacific
Celgar Pulp
Temple-Inland
Weyerhaeuser
Weyerhaeuser
Weyerhaeuser
Willamette
Augusta, GA
Windsor, Que.
Longview, WA
Port Mellon, B.C.
Columbus, MS
Peace River, Alta.
Hinton, Alta.
Augusta, GA
Courtland, AL
Eastover, SC
Port Wentworth, GA
Clairborne, AL
Demopolis, AL
Silsbee, TX
Savannah, GA
Boyle, Alta.
Castlegar, B.C.
Silsbee, TX
Longview, WA
Plymouth, NC
Plymouth, NC
Johnsonburg, PA
HW/SW
HW
sw
sw
sw
HW/SW
SW
HW
SW
HW
HW/SW
SW
HW/SW
HW
SW
HW/SW
SW
HW
HW/SW
HW/SW
HW/SW
HW
770
1145
1060
1450
1575
1360
1460
1520
1180
1250
865
1620
865
1085
2450
2020
1605
1250
1455
700
1100
780
1988
1988
1988
1990
1990
1990
1990
1991
1991
1991
1991
1991
1992
1992
1992
1993
1993
1994
1994
1994
1994
1994
Page 4-17
-------
Pollution Prevention in Pulp &. Paper
Section Four - Pulping
TABLE 4-1 (cont)
Installations of Extended Delignification Systems Worldwide
Company
Location
Fiber
Furnish
Capacity
(tons/day)
Start-
Up
MCC®/EMCC® Retrofits of Kamyr Continuous Digesters
North American Mills
Consolidated Papers
International Paper
International Paper
Champion Intl
Champion Intl
E.B.Eddy
Georgia Pacific
Georgia Pacific
Northwood
Union Camp
James River
Wisconsin Rapids ,WI
Georgetown, SC
Mobile, AL
Courtland, AL
Quinnesec, MI
Espanola, Ont.
Ashdown, AR
New Augusta, MS
Prince George, B.C.
Savannah. GA
Camus, WA
SW
SW
HW/SW
HW
HW
SW
SW
HW/SW
SW
SW
HW
500
1300
1200
900
1000
525
900
1800
1100
2450
580
1987
1990
1990
1990
1990
1991
1991
1991
1991
1992
1993
New Kamyr MCC®/EMCC® Continuous Digesters
European, Asian, South American Mills
Metsa-Botnia
Korsnas
Kemi Oy
Iggesund Paperboard
Oji Seishi
Nagoya Pulp
Oji Seishi
Celulosa Arauco
Celulosa Pacifico
Aanekoski, Finland
Gavle, Sweden
Kemi, Finland
Iggesund, Sweden
Kasugai, Japan
Kanishi, Japan
Yonago, Japan
Arauco, Chile
Mininco, Chile
HW/SW
HW/SW
HW/SW
SW
SW
HW
HW
SW
sw^
1210
1160
800
690
800
800
1200
1210
1220
1985
1988
1988
1988
1989
1990
1991
1991
1991
Page 4-18
-------
Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-1 (cont)
Installations of Extended Delignification Systems Worldwide
Company
Location
Fiber
Furnish
Capacity
(tons/day)
Start-
Up
MCC®/EMCC® Retrofits of Kamyr Continuous Digesters
European, Asian, South American Mills
Enso Gutzeit
NCR
Stora
Iggesund Paperboard
Varkaus, Finland
Vallvik, Sweden
Skutskar, Sweden
Iggesund, Sweden
HW
SW
HW
HW
590
515
420
800
1983
1990
1990
1990
Beloit RDH Batch Extended Cook Systems
North American Mills
Packaging Corp
S.D. Warren (Scott)
Bo water
Fletcher Challenge
Willamette
Valdosta, GA
Westbrook,ME
Calhoun, TN
Crofton, B.C.
Bennettsville, SC
HW/SW
HW/SW
HW/SW
SW
HW/SW
1,000
450
1200
775
900
1984
1989
1990
1990
1990
Beloit RDH Batch Extended Cook Systems
Rest of World
Joutseno Pulp
Nymolla
Cellulosas de Naviron
Chung-Hwa
Joutseno, Finland
Nymolla, Sweden
Durango, Spain
Hualien, Taiwan
HW/SW
SW
SW
HW
950
860
350
400
1986
1987
1989
1992
Sunds SuperBatch Batch Extended Cook Systems
Worldwide
Jefferson Smurfit
ASSI
Mondi Paper
Sodra
Enocell
Jacksonville, FL
Karlsborg, Sweden
Richard's Bay, S. Africa
Varo, Sweden
Uimaharju, Finland
SW
SW
HW/SW
SW
HW/SW
690
875
1500
900
1800
1990
1984
1984
1988
1993
Source: MacLeod (1992).
Page 4-19
-------
Pollution Prevention in Pulp & Paper Section Four - Pulping
Some mills may retrofit their existing pulping equipment without the input of the vendor
companies discussed above. While technically not MCC®, EMCC®, or RDM, the equipment and principles
applied are the same.
4.2.2 Costs and Economics
Capital costs for extended delignification equipment have been quoted at $15 to $16 million for
a new, 1,200 air dried short tons (ADST) per day MCC® digester (Kamyr, Inc. 1990; cited in EPA, 1990a)
with the installed cost closer to $45 million.
Extended cooking can be retrofitted to most continuous digesters. Based on experience with mills
in North America, McCubbin (1992) suggests there are three categories of retrofits, each having a different
baseline set of conditions and subsequent upgrade costs:
(1) Digesters operating with an upflow of wash water and a dilution factor of at least 0.5.
This generally applies to digesters installed after 1980 that were not equipped for
extended cooking. These require only a simple "wash zone" retrofit, costing under $1
million dollars in most cases;
(2) Overloaded conventional Kamyr digesters, or two-vessel digesters that are unsuitable for
a wash-zone retrofit. These would have to be replaced with new digesters. The few
continuous digesters in North America that are not of Kamyr manufacture, for which there
is no demonstrated technology for retrofitting extended cooking, are also in this category.
Capital costs for replacement would be in the $30 to 40 million range.
(3) Most other continuous digesters would require conversion to a two-vessel installation by
adding another vessel downstream of the existing one, and retaining the existing vessel
and its chip feed system to perform the functions of an impregnation vessel. This
normally allows an increase in digester capacity and improved pulp washing. Costs
would be in the $15 to 30 million range for existing digesters.
In a study prepared for the paper industry, Phillips et al. (1992) suggest that costs for retrofit of
a model 1,320 tpd continuous digester currently running at 50 percent chlorine dioxide substitution average
$4.9 million, assuming no need to upgrade recovery capacity. Annualized over 20 years at an 8 percent
discount rate, the incremental annual capital costs per ton are $1.08, but these are more than offset by
operating cost savings (energy and bleaching chemicals) of $6.19 per ton.
Page 4-20
-------
Section Four - Pulping Pollution Prevention in Pulp & Paper
Costs for new RDH systems have been quoted at around $1.5 million for each new batch digester
plus $5 million for the accumulator tank farm, with a turnkey system running approximately three times
that amount or around $35 million for a 5-digester system (Beloit Corp. 1990; cited in EPA, 1990a).
Costs for converting an existing pulping system to RDH would likely range around $0.5 million per
digester. Direct steaming digesters would be much more expensive to retrofit compared to those with
liquor circulation systems.
The retrofit potential for existing batch systems in the United States is somewhat limited due to
the age of the digesters. Most U.S. batch digesters are of 1940 to 1970 vintage and hence many would
not be suited for the complex modifications required for application of RDH. Space requirements for the
RDH tank farm may also limit their applicability at some mills.
Beloit, the vendor of the RDH system, has reported that the expected payback period on an RDH
retrofit is approximately 18 months. Among the benefits cited are: (1) steam savings, (2) better pulp
quality, (3) higher solids concentration in the liquor, leading to lower evaporation costs, and (4) savings
in bleaching chemical costs of approximately 50 percent.
4.2.3 Pollution Prevention Potential
Extended delignification has been shown to reduce the kappa number of brownstock softwood pulp
from a range of 30 to 32 for conventional pulping to a range of 12 to 18. Target ranges, however, are
currently around 20 to 25 (Stromberg, 1993). Hardwood kappas can be reduced from around 20 to a
range of 8 to 10, with current targets generally around 12 to 15. Below these target ranges, at this time
at least, yield losses may accelerate and pulp quality may be diminished. Ongoing refinements to these
processes are expected to bring these kappa numbers down further, however, over the next few years.
The impact of brownstock kappa number reductions on bleaching chemical demands and formation
of many chlorinated organics is now well-documented. AOX and polychlorinated phenols will be reduced
in approximate proportion to reductions in the brownstock kappa number. Declines in conventional
pollutants such as BOD5, COD, and color from MCC® processes have also been widely realized.
According to Heimburger et al. (1988a), MCC® cooking reduced kappa by 22 percent, while BOD5 and
color declined by 29 and 31 percent, respectively (see Table 4-2).
Page 4-21
-------
Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-2
Characteristics of Conventional
and MCC Pulps
Parameter
Kappa number
BOD, mg/dm3
COD, mg/dm3
TOC, mg/dm3
TOC1, mmol/dm3
Color, Pt mg/dm3
Conventional
32
675
4,618
1,579
1.93
10,202
MCC
25
480
3,931
1,220
1.45
7,065
Percentage
Decrease
21.9%
28.9%
14.9%
22.7%
24.9%
30.7%
BOD = biochemical oxygen demand.
COD = chemical oxygen demand.
TOC = total organic carbon.
TOC1 = total organic chlorine.
Source: Heimburger et al. (1988a).
Page 4-22
-------
Section Four - Pulping Pollution Prevention in Pulp & Paper
Due to the higher level of effluent recycle and recovery of solids from effluent achieved with
extended delignification, the mill's chemical recovery system may experience an increase in load and there
may be an increase in byproducts from the recycle system following implementation. Additional energy
may be required to evaporate the increased effluent volume passing through the recovery boiler (although
this will be more than offset by the heat recovered from the additional solids). Incineration of higher
solids volumes will cause some increase in air emissions and solid wastes (recovery boiler ash), although
these increases are expected to be minor.
4.2.4 Compatibility with Downstream Bleaching Stages
When used in combination with oxygen delignification (see Section 4.3), kappa number reductions
to the range of 6 to 10 have been reported for softwoods (Galloway et al., 1989; Shin et al., 1990). Pulps
in this range are extremely bleachable and could be brought to high brightness using elemental chlorine-
free (ECF) and possibly totally chlorine-free (TCP) sequences.6
Data in Table 4-3 show the pollution reduction potential of extended delignification in combination
with oxygen delignification (case C), as compared with conventional cooking (case A) or oxygen
delignification alone (case B). In terms of organics discharge, the data show that oxygen delignification
alone will decrease AOX from 7.9 to 4.7 kg per ton, while oxygen plus extended delignification decreases
it to 3.6 kg per ton. A high chlorine dioxide substitution (70 percent or higher) can be further effective
in reducing AOX to a level of 1.9 kg per ton (see Section 5.2). Table 4-3 also shows beneficial impacts
of MCC® pulping and oxygen pre-bleaching on BOD5, COD, and color. When extended cooking is
combined with an oxygen stage, BOD5 falls from 28 to 20 kg per ton, COD falls from 100 to 55 kg per
ton, and color declines from 300 to 80 kg per ton.
Table 4-4 compare the characteristics of conventional and RDH pulps, both with and without an
oxygen prebleaching stage (Heimberger et al., 1988a). Total active chlorine (percent on pulp) is reduced
6 ECF pulps are produced using 100 percent chlorine dioxide substitution for chlorine; no elemental
chlorine or hypochlorite are used. In TCP sequences, chlorine dioxide is also eliminated. (Note that no
bleached kraft mills in the U.S. are currently producing high brightness TCP pulps on a sustained basis,
although Louisiana-Pacific has committed to do so at its Samoa, California mill by 1995 as part of a
consent decree signed with EPA.)
Page 4-23
-------
Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-3
Impact of Modified Continuous Cooking
on Effluent Characteristics
Process
Case A
CaseB
CaseC
CaseD
Flow
nrVadmt
50-55
50-55
50-55
50-55
BOD5
kg/admt
28
22
20
20
COD5
kg/admt
100
70
55
55
Color
kg/admt
300
100
80
65
AOX
kg/admt
7.9
4.7
3.6
1.9
Case A State of the art conventional kraft mill.
Case B - Case A with oxygen delignification.
Case C - Case B plus modified continuous cooking (ED).
Case D - Case C with 70% chlorine dioxide substitution.
Source: Galloway et al. (1989).
Page 4-24
-------
Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-4
Characteristics of Conventional
and RDH Pulping
Parameter
Kappa No.
Total Active C12, %
o.d. pulp
Brightness, % ISO
COD, kg/ton
BOD, kg/ton
Conventional
No O2 Stage
34
11.7
89
78
14
O2 Stage
23
7.5
90
57
11
RDH Pulping
No O2 Stage
23
9.0
88
50
10
O2 Stage
15
5.3
88
40
8.5
Source: Heimburger et al. (1988a).
Page 4-25
-------
. D , . P ,r Section Four - Pulping
Pollution Prevention in Pulp & Paper _ .
from 11.7 to 9.0 without oxygen and from 7.5 to 5.3 when oxygen prebleaching is practiced. Reductions
in chlorine usage will generally lead to proportionate decreases in the formation of chlorinated organics.
Table 4-5 shows results from a series of laboratory investigations conducted by the Beloit Corp.
and researchers at North Carolina State University (Shin et al., 1990). Conventional kraft pulps and pulps
produced using the RDH extended delignification process were dehgnified using conventional techniques,
as well as with pressurized oxygen (O) and atmospheric oxygen (E0). For the conventional pulp,
measured AOX levels were 3.1 kg per ton (Case 4). Pulping with RDH decreased the AOX level to
between 1.28 and 1.40 kg per ton, or a decrease of 55 to 59 percent.
When conventional pulp was bleached with atmospheric oxygen, AOX fell to between 2.7 and
2.8 (Cases 2-3), while bleaching with pressurized oxygen reduced AOX to 1.74 kg per ton (Case 1).
Thus, RDH by itself had a greater impact on AOX than oxygen.
In cases 7 through 11, the effectiveness of combining RDH and OD is seen. Bleaching the RDH
pulps with atmospheric oxygen reduced the AOX levels to a range of 0.90 to 0.95 kg per ton (Cases 7-9),
while pressurized oxygen produced even lower results of 0.54 to 0.60 per ton (Cases 10-11). In cases 12-
15, chlorine dioxide substitution was increased from 75 to 100 percent, resulting in still greater decreases
in AOX (to as low as 0.44 kg per ton in Case 15). Complete substitution of C1O2, however, has been
shown to result in less efficient delignification (Reeve, 1990). Continued improvement hi effluent can be
obtained, but it will come at the expense of some yield.
Researchers from Sunds recently dehgnified Southern pine to kappa numbers between 34.4 and
14.1 using an industrial SuperBatch™ digester (Norden et al., 1992). Each sample was then oxygen
dehgnified, resulting in a 50 percent reduction in kappa number. Using 100 percent C1O2 substitution,
the 34.4 kappa pulp (oxygen-delignified to 15.2) consumed 48 kg active chlorine per ton during laboratory
bleaching to 89 percent ISO brightness. The 14.1 kappa pulp, (oxygen-delignified to 7.7), consumed 30
kg active chlorine per ton. AOX levels of these two pulps were a very low 0.35 and 0.1 kg per ton,
respectively.
In the same trials, two oxygen dehgnified pulps with kappa numbers 9.9 and 7.7 were bleached
using TCP sequences OXP and OZXP, where X denotes treatment with EDTA and Z is ozone
delignification (see Section 4.4). The 7.7 kappa pulp was bleached to 80 percent ISO with a peroxide
Page 4-26
-------
Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-5
Impact of Rapid Displacement Heating Cooking
Techniques on Effluent Characteristics
Pulping
Sequence
Conventional
Kraft Pulping1
(kappa no. 33)
RDH Kraft
Without
Oxygen1
(kappa no. 16)
RDH Kraft
With Oxygen1
(kappa no. 16)
100% Chlorine
Dioxide
Substitution
(kappa no. 16)
Bleaching
Case
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Sequence
O(DC)(EO)D
(EO)(DC)(EO)D
(EO)(DC)(EO)(De)D
(DC)(EO)(De)D
(DC)(EO)(D)
(DC)(EO)(DE)D
(EO)(DC)(EO)D
(EO)(DC)(EOP)D
(EO)(DC)(EO)(De)D
O(DC)(EO)D
O(DC)(EOP)D
(EO)(E)(EO)D
(EO)(D)(EOP)D
OD(EO)D
OD(EOP)D
Color
Trial A
44
68
68
120
—
62
47
36
47
27
20
32
56
18
13
Trial B
44
68
68
120
—
60
47
36
47
27
20
32
26
18
13
AOX
Trial A
1.66
2.71
2.80
—
1.27
1.32
0.92
0.90
0.93
0.60
0.54
0.57
0.64
0.56
0.44
Trial B
1.74
2.70
2.80
3.13
1.28
1.40
0.92
0.94
0.95
0.59
0.55
0.57
0.64
0.58
0.50
1C\O2 substitution
Source: Shin et al.
75% for all bleaching sequences
(1990).
Page 4-27
-------
D i jt v™»r Section Four - Pulping
Pollution Prevention in Pulp & Paper .
consumption of 30 kg per ton. When ozone was added to the sequence, a brightness of 89 percent ISO
was achieved with the same peroxide consumption. In full scale production, this sequence would be
expensive (due to the costs of peroxide) but possibly competitive.
Experiments with RDM pulps have also been performed using ECF and TCP sequences (Kumar
et al., 1992). Various bleaching sequences involving several pretreatments and additions were
investigated. Using oxygen delignification to reduce kappa by 50 percent, a brightness level of 85 to 87
percent ISO could be achieved using as little as 0.6 percent C1O2. AOX levels were not reported but are
expected to be quite low. TCP sequences using ozone produced 85 ISO brightness pulps.
4.2.5 Impacts on Other Aspects of Mill Operations
Recovery Boiler Operations
Extended delignification increases the amount of lignin and organic solids removed during the
kraft cooking process. As indicated earlier, the black liquor washed from the pulp is concentrated and
burned in the recovery furnace. Boilers running at or near capacity (in terms of solids handling capability)
may be unable to accommodate the additional solids without further process modifications. A range of
options are available, however, for increasing solids handling by 5 to 10 percent, and retrofits and rebuilds
can boost capacity by significantly more at less than the $50 to $100 million cost of a new recovery
boiler. Many mills already practice some of these techniques:
Additional evaporator - Additional evaporation stages will increase the concentration of the black
liquor, resulting in improved combustion (reduced gas flow) as well as lower sulfur emissions.
Although this is a common upgrade option, the higher consistency solids are more difficult to
handle and will necessitate improved pumping and firing equipment;
Transport black liquor solids offsite for disposal - Where other kraft mills with excess recovery
boiler capacity are within 500 miles of the mill, it may be feasible to ship additional solids offsite
for firing. This has become common practice in some areas of North America and Europe;
Reduce boiler load per ton of solids Although boiler load is discussed in terms of pounds of
solids burned, in practice the capacity ultimately depends on the heat content of the black liquor
solids:
(1) In at least one Swedish mill, black liquor oxidation has been used to reduce heat value
of liquor solids sufficiently to accommodate an 8-10 percent increase in capacity;
Page 4-28
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Section Four - Pulping Pollution Prevention in Pulp & Paper
(2) Mills can separate soap from the liquor solids for incineration or sale offsite. Soap
removal can reduce the heat value of the liquor solids by 4 to 8 percent, thereby enabling
further capacity increases;
Increase black liquor storage capacity Some mills may lack sufficient black liquor storage
capacity, resulting in insufficient supply to keep the boiler operating steadily at capacity (essential
for efficient operation). By constructing additional supply capacity the mill may be able to obtain
a higher liquor solids throughput from the existing boiler;
Black liquor gasification A relatively recent technology, this proprietary process involves
gasification of strong black liquor in a closed vessel. This produces a smelt similar to that from
a conventional recovery furnace. A single Chemrec® unit installed at the Swedish Frovifors mill
has boosted the recovery capacity by 3 tons dry solids per hour, the equivalent of 15,000 metric
tons per year of pulp (Gotavarken, 1993).7 Unlike other boiler modifications, the black liquor
gasification unit can be installed without any lost production time.
Anthraquinone addition in pulping The use of an anthraquinone catalyst in the digester can
increase the yield from kraft pulping by up to 2.5 percent (see Section 3.4) and decrease the
production of black liquor solids by up to 6 to 10 percent;
Reduce boiler water feed temperature and/or temperature of combustion air If steaming
rate is the limiting factor on boiler capacity, these actions can reduce the steaming rate by several
percent;
Enrich combustion air with oxygen - For mills limited by gas flow, it may be possible to boost
boiler load capacity by introducing oxygen into the feed air;
Add incremental boiler capacity Clement (1993) has recently presented several case studies
that illustrate a variety of options for increasing boiler capacity incrementally through rebuilds and
equipment upgrades (see Table 4-6). In the four U.S. projects cited, capacity increases ranged
from 10 to 63 percent. The costs have ranged from $3 to $63 million;
Boiler rebuild/replacement - Recovery boilers are among the most complex and expensive pieces
of equipment at the pulp mill. The costs of constructing a new boiler could range from $50 to
100 million. While this may seem exceedingly costly, for an older mill a modern boiler will bring
substantial additional benefits in the form of greater efficiency, easier maintenance, and reduced
air pollution;
Production penalty Mills may choose to decrease production to accommodate the additional
solids per ton of pulp that results from oxygen or extended delignification. This operational
penalty, however, is generally considered excessive.
7 The Chemrec® process is offered by the Swedish company Gotavarken (U.S. subsidiary located in
Charlotte, North Carolina).
Page 4-29
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-6
Sample Boiler Upgrade and Rebuild Projects
Babcock & Wilcox Company
Company
Mill Location
Description of project
Stone Container
Florence, South
Carolina
The recovery boiler was modified to achieve an
increase in solids throughput of 10 percent. The
air feed and liquor firing systems were
substantially upgraded.
Potlatch Corporation
McGehee, Arkansas
The furnace at this mill was enlarged and a new
three-level air system was added. Solids
capacity was increased from 680 tons per day to
1,111 tons per day, an increase of 63 percent
Gaylord Container
Bogalusa, Louisiana
The boiler, already running at 22 percent over
capacity, was upgraded to reduce the time
between waterwashing and to cut TRS. After
the rebuild, the mill was able to run at 28
percent above capacity with reduced TRS. Time
between waterwashings was increased from six
weeks to six months.
James River
St. Francisville,
Louisiana
Conversion of the boiler to low odor
configuration resulted in an increase in solids
processing capacity to 1,224 tons per day from a
level of 998 tons per day (design capacity was
only 816 tons per day). In addition to upgrading
the combustion, heating, and liquor firing
systems, seven feet was added to the furnace
depth and nine feet was added to the height.
Source: Clement (1993).
Page 4-30
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Section Four - Pulping Pollution Prevention in Pulp & Paper
In addition, it is always possible (though less preferable) for the mill to discharge the effluent containing
any additional solids that cannot be accommodated in the recovery boiler. The solids in this effluent were
previously removed during the bleaching stages and could not be recycled. Thus, there would be no net
increase in solids discharged to treatment, but the ratio of chlorinated to unchlorinated solids would
decrease.
Air and Solid Waste Emissions
The kraft liquor recovery system produces air emissions (from the recovery furnace) and solid
wastes such as ash and precipitates (from the recovery boiler), dregs (from the dregs washer) and grits
(from the lime slaker). The volume of air emissions is related, all else being equal, to the volume of
solids processed in the recovery boiler. Thus, increases in solids recovery through the recovery boiler
could result in increased air emissions and solid wastes.
The modern recovery furnace is equipped with sophisticated air emissions control equipment such
as electrostatic precipitation (ESP). Precipitates (primarily sodium sulfate and sodium carbonate) are
returned to the liquor makeup system, hence the only material losses are the ash, dregs and grits. These
are usually landfilled. A higher degree of delignification would probably result in a minor (i.e., less than
5 percent) increase in the quantity of these materials going to the landfill
Energy Requirements
Extended delignification will cause an increase in steam demand (due to the longer cooking
period), however more energy will be recovered from the additional lignin solids. The greatest energy
impacts may be observed indirectly though, through reductions in bleaching chemical demands. Most
bleaching agents are manufactured by applying large amounts of energy to raw inorganic minerals (such
as chloride to manufacture sodium chlorate, caustic, and chlorine). McCubbin (1993) has modeled the
onsite and offsite energy impacts of a variety of pollution prevention measures. Conversion of his model
1,000 tpd CpEDED mill to extended delignification would result in no net increase in onsite power
requirements, but would reduce offsite power requirements (through decreased chemical use) by 3.4 MW
(or around 800 kWh per ton).
Page 4-31
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Pollution Prevention in Pulp & Paper
Pulp Yield
Reductions in brownstock lignin content below the current target ranges of 20 to 25 on softwood
and 12 to 15 on hardwood are possible using extended delignification. Full-scale mill trials have produced
pulps with kappas below 10 (hardwood) and 15 (softwood), with no discernible effect on strength
properties (Elliott, 1989; Whitley, 1990). With present technology, however, yield losses will begin to
occur at these levels. Yields on softwood pulped drop from 45.0 to 43.5 percent when kappa is reduced
from 20 to 15 (Gullichsen, 1991). As the technology develops further these results may improve.
At the same time, conventional digesters currently suffer a pulp yield loss of around 3 percent due
to knots and rejects removed during post-digester screening. About half the mills grind these and
reprocess them, at a cost, while the other half landfill them. In extended cooking, rejects are under 0.5
percent, hence any yield loss is significantly offset.
43 OXYGEN DELIGNIFICATION
Oxygen delignification provides an additional way to extend the pulp cooking process, thereby
lowering the bleaching chemical demands and the amount of pollution associated with chlorine-based
bleaching stages.8 The technique involves the integration of an oxygen reaction tower in between the kraft
pulping stages and the bleach plant. The brownstock pulp from the digester is first washed and then
mixed with oxygen and sodium hydroxide as it enters the pressurized reactor. There, the pulp undergoes
oxidative delignification. The pulp is then washed again to remove additional dissolved lignin solids
before proceeding to the bleaching line.
Installation of an oxygen delignification stage can reduce the kappa number of brownstock pulp
from a range of 30 to 35 to perhaps 16 or 17 (i.e., a 50 percent reduction). At these kappa levels, a range
of bleaching options are available. At a minimum, chlorine requirements will fall in approximate relation
to the percent reduction in kappa number. When used in combination with other process modifications,
8 The term "oxygen bleaching" is sometimes used to refer to oxygen delignification, because it can
be viewed as a replacement for chlorine in the first bleaching stage. Oxygen bleaching is also used by
some to describe the addition of elemental oxygen to the caustic extraction stage, i.e., oxidative extraction
(E0). In this report, however, we avoid the term oxygen bleaching entirely.
Page 4-32
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Section Four - Pulping Pollution Prevention in Pulp & Paper
oxygen delignification can reduce the requirements for chlorine-based bleaching agents still further, and
under certain circumstances eliminate them completely.
High-Consistency Versus Medium-Consistency Systems
Two different types of oxygen systems have been designed and are in use around the world. The
systems differ in terms of the reaction consistency of the pulp. In high consistency (HC) systems, pulp
is reacted at 25 to 28 percent consistency, while in medium consistency (MC) systems the pulp
consistency is typically between 10 and 12 percent. MC systems can take pulp directly from the
brownstock washers while in HC systems a press is required to remove excess water from the pulp prior
to reaction. Table 4-7 presents typical operating data for HC and MC oxygen systems.
The earliest oxygen delignification installations were of the high consistency type. Initially, it was
believed that the higher consistencies promoted greater absorption of oxygen by the pulp fibers, leading
to a higher degree of delignification. More oxygen is needed, however, and better mixing is required to
achieve the same thoroughness and uniformity of reaction obtained in a medium consistency reactor.
Figures 4-8 and 4-9 show process flows and equipment used in an HC oxygen delignification
process. In Figure 4-8, the process begins with cooked pulp which is taken from the brownstock washers
at about four percent consistency. A small amount of magnesium salts (MgSO4) are blended with the pulp
in a mixing chest (2 to 3 Ibs per ton pulp). The magnesium additive has been found to protect the
cellulose fibers from oxidative degradation that may occur in pockets of high oxygen concentration inside
the reactor. Thus, the magnesium permits a further degree of delignification without any loss of yield.
Following the addition of magnesium, the pulp is dewatered in a press to the desired consistency,
about 25 to 28 percent. Fresh caustic solution or, more commonly, oxidized white liquor, is added at the
press discharge. The pulp then passes through a fluffer which shreds the pulp. Shredding exposes more
pulp surface area and promotes a more consistent reaction. The fluffed pulp is then deposited on the bed
of the oxygen reaction vessel, steamed to around 90 to 120° C, and injected with gaseous O2. The pulp
is swept down a serious of trays inside the reactor by sets of rotating arms (see diagram in Figure 4-9).
Gases generated during the lignin oxidation must be purged from the reactor to avoid a fire or explosion
Page 4-33
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-7
Typical Operating Data for Oxygen Delignification
of Kraft Softwood Pulp
Parameter
Pulp consistency, %
Delignification, %
Retention time, min.
Initial temp., °C
Pressure, kPa
Inlet
Outlet
Steam consumption,
kg/metric ton
Low pressure (450 kPa)
Medium pressure (1,140 kPa)
Evaporator (450 kPa)
Power consumption,
kWh/metric ton
Alkali consumption,
kg/metric ton
Oxygen consumption,
kg/metric ton
Magnesium ion,
kg/metric ton
High
Consistency
25-28
45-50
30
100-105
500-600
500-600
—
75-100
30-50
40-50
21-23
20-24
0.5
Medium
Consistency
10-12
40-45
50-60
100-105
700-800
450-500
70
200-300
90-100
35-45
25-28
20-24
0.5
Source: Tench and Harper (1987).
Page 4-34
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
Vent
Post Oxygen
Washer
Figure 4-8. Process flow for high-consistency (HQ oxygen delignification.
Source: Miller, 1992.
Page 4-35
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
Press
Flutter
Pulp at 4%
consistency
Pump
Discharger
Dilution
Diluted Stock Out
ROTATION (CLOCKWISE)
DRIVE SHAFT
SPEED SWITCH
BEARING HOUSING
MECHANICAL SEAL
STOCK INLET
•TOCK M KOW TAM
STOCK INLET
TOOTHED SCflEW
Oxygen reactor
Figure 4-9. Equipment diagrams for high-consistency oxygen delignification.
Source: Miller, 1992.
Page 4-36
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Section Four - Pulping Pollution Prevention in Pulp & Paper
hazard.9 The reacted pulp is diluted with post-oxygen filtrate to about six percent consistency and is then
discharged to a blow tank, where the pulp is restored to atmospheric temperature and pressure. Post-
oxygen washing stages are usually added to maximize the amount of solids removed prior to chlorinafion.
Good pulp washing is the key to effective implementation of oxygen delignification.
The thickness of the stock in an HC system can lead to flow problems in the reactor and may
create pockets of elevated oxygen concentrations. With a buildup of oxygen, malfunctions due to pump
failures or blockages could present a risk of fire or explosion.
Medium consistency or MC oxygen systems operate at a lower solids concentration of
approximately 10 to 14 percent. This consistency can be delivered directly from the brownstock washers,
thereby eliminating the need for the pulp press. An equipment diagram for an MC system is shown in
Figure 4-10. Note that the diagram of the MC system shows the upflow design of the reactor. Pulp is
fluidized in the high shear mixers and then travels upwards through the reactor vessel. This contrasts with
the HC systems, in which the pulp enters at the top or the reactor before being swept down towards the
bottom.
In the MC process, the pulp is blended with oxygen using high shear mixers or pumps, which
cause the pulp slurry to fluidize and the oxygen to form very fine bubbles. The development of the high
shear mixer has allowed for faster and more uniform reactions between the oxygen and the lignin.
Retention time is approximately 60 minutes for hardwood and 45 to 60 minutes for softwood. Research
so far indicates that single-stage MC systems are capable of slightly less delignification than HC.
Nevertheless, the MC system has several advantages: (1) no requirement for dewatering equipment, (2)
reduced safety hazard, and (3) reduced potential for pulp degradation, which can reduce or eliminate the
need for magnesium viscosity additives. For these reasons, the medium consistency system is now being
recommended by vendors for most new oxygen installations (McCubbin at al., 1991), and accounts for
almost all of the recent U.S. installations.
9 These gases are generally considered safe and are normally vented to the atmosphere, although there
is some concern that these may contain VOCs such as methanol or acetone.
Page 4-37
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
CLOVE-ROTOR®
Pump
Mixer
Post Oxygen
Filtrate
Figure 4-10. Process flow for medium-consistency (MC) oxygen delignification.
Source: Miller, 1992.
Page 4-38
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Section Four - Pulping Pollution Prevention in Pulp & Paper
Two-Stage Oxygen Systems
One of the more recent developments has been the introduction of multiple-stage oxygen systems
to further increase the delignification effectiveness. At least a dozen mills around the world, including
two in the United States and two in Canada, now use two-stage oxygen delignification. Data from
Johnson (1993) indicate that 2-stage systems account for eleven percent of oxygen capacity in 1993. This
modification requires installation of two reactor vessels with additional mixing equipment installed in
between. Lignin removal of between 60 and 65 percent has been reported for 2-stage systems (Deal,
1991), as opposed to 50 percent reductions with a single stage system.
4 J.I Number of Installations
Oxygen delignification has been commercially available since the late 1960s, having been
developed in Sweden and first applied successfully in a South African mill in 1970. The more widespread
adoption of oxygen came with the development in France of magnesium additives that inhibited
degradation of cellulose, a problem that had afflicted earlier implementation attempts.
Until recently, oxygen delignification had been more widely adopted outside of North America.
In recent years, the adoption rate in the U.S. and Canada has increased. According to Johnson (1993),
there are presently 155 mills worldwide operating oxygen delignification systems, representing 26 million
tons of annual production or 34 percent of total world kraft output (see Figure 4-11). During 1992, 32
additional systems started up, and another 20 more systems have been sold and/or are under construction
as of March, 1993.
Oxygen is currently installed or planned for 27 U.S. mills. Of these, 16 will have come online
since 1989. U.S. capacity is currently around 8.1 million tons per year. Figure 4-11 illustrates the rapid
rate of adoption of oxygen systems worldwide and in the United States, and Table 4-8 lists these
installations.
Page 4-39
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
Figure 4-11
Installations of Oxygen Delignification Systems
World
144
148
Number of Systems Installed
114
94
79
63
1 1 3
6699
12
15 17
21
25 25
30
34
40
120
100
60
40
20
-d
-------
Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-8
U.S. Installations of Oxygen Delignification Systems
Company Name
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Chesapeake
Weyerhaeuser
Union Camp
Union Camp
Consolidated Paper
Champion International
Champion Internationa]
Flambeau Paper
Bo water Corp.
Louisiana-Pacific
Willamette Industries
Champion International
Wausau Paper
Westvaco
Weyerhaeuser
Alabama Pine
Champion International
Confidential client
Simpson Paper
Union Camp
Weyerhaeuser
Weyerhaeuser
Champion International
Champion International
Champion International
Potlatch Corp.
Union Camp
Location of Mill
W. Point, VA
Oglethorpe, GA
Franklin, VA
Eastover, SC
Wisconsin Rapids, WI
Pensacola, FL
Pensacola, FL
Park Falls, WI
Calhoun, TN
Samoa, CA
Bennettsville, SC
Quinnesec, MI
Brokaw, WI
Covington, VA
Columbus, MS
Clairborne, AL
Courtland, AL
Southern U.S.
Eureka, CA
Eastover, SC
Cosmopolis, WA
New Bern, NC
Canton, NC
Canton, NC
Courtland, AL
Lewiston, ID
Franklin, VA
Production
Capacity
(tpd)
550
1,000
800
650
500
800
600
200
1,300
750
840
1,150
290
915
1,400
1,415
1,150
1,385
850
1,100
540
1,080
660
700
1,245
1,130
900
Consist-
ency [a]
HC
HC
HC
HC
MC
MC
MC
MC
MC
MC
MC
MC
MC
MC
MC
MC
MC
MC
MC
HC
MC
MC
MC
MC
MC
HC
HC
Year of
Startup
1972
1980
1981
1984
1985
1986
1987
1987
1988
1988
1988
1989
1989
1990
1990
1991
1991
1991
1991
1991
1991
1991
1992
1992
1992
1992
1992
Total capacity 23,900
[a] HC = High consistency, MC = Medium consistency
Source: Johnson (1992).
Page 4-41
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Pollution Prevention in Pulp & Paper ___ Section Four ' PulPinS
Costs and Economics
The capital costs of the oxygen reaction tower and related equipment (pumps, washers) have been
estimated at between $8 and $16 million (see Table 4-9). 10 At the low end of this range, the savings in
chemical costs alone, plus savings on current or future effluent treatment requirements, may favor the
switch to oxygen. At other mills considerably greater investment may be required, particularly to upgrade
pulp washing equipment. Successful implementation of oxygen delignification requires effective pulp
washing both in front of and following the oxygen stage, to avoid excess oxygen consumption and
minimize potential for heat generation. For mills that require a considerable upgrade in pulp washing
equipment, the cost of conversion may be closer to $20 to $25 million.
Installation of an oxygen delignification stage can help mills avoid the costs of installing a new
chlorine dioxide generating system or replacing an aging C-stage in their bleaching line. By lowering the
pre-bleaching kappa number by 50 percent, oxygen will permit some mills to shorten their bleach sequence
(i.e., eliminate a C-stage) and/or increase the chlorine dioxide substitution rate without installing additional
C1O2 generating capacity.11 By taking some of the delignification load off of the bleach plant, oxygen
stages can also alleviate bleach plant capacity bottlenecks.
Historically, oxygen has been generated offsite and has been expensive for use as a bleaching
agent. In North America, most oxygen for industrial applications is generated using cryogenic air
separation (or fractional distillation), with nitrogen and argon produced as coproducts (SRI International,
1988). Cryogenic techniques produce oxygen in the purest form. Installations are large hi scale and
located in regions of heavy demand. Most mills today using oxygen for bleaching will generate it onsite
using non-cryogenic systems provided by equipment vendors. Typically, the mill enters into a 10- to 20-
year "over the fence" supply contract with the vendor. The mill supplies the land for construction of the
10 In a study prepared for the paper industry evaluating the impacts of adopting pollution prevention
options, Phillips et al. (1993) used an average cost of $37.7 million for the oxygen equipment alone. As
seen, these costs are considerably higher than those reported elsewhere. The source for the equipment cost
estimates is reported only as "national industry averages."
11 The conventional approach to increasing the chlorine dioxide substitution rate involves boosting
C1O2 generating capacity and increasing the ratio of C1O2 to C12. Alternatively, a mill can add an oxygen
stage to first decrease the overall amount of bleaching chemicals required. Then, using existing C1O2
generating capacity, they cut back on C12, effectively raising the C1O2 substitution rate. In the latter case,
the mill obtains the added benefit of being able to recycle the effluent from the O-stage and to decrease
the flow of effluent going to the wastewater treatment plant.
Page 4-42
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-9
Capital Cost Estimates for Oxygen Delignification Systems
Information
Source
Literature
Supplier
Consultant
EPA (1990a)
Phillips et al.
(1992)
Idner (1988)
Description
Short sequence
OD system
OD
OD
MC
Hardwood MC
Softwood HC
Louisiana-Pacific,
Samoa, CA, MC
Simpson Paper,
Fairhaven, CA, MC
Weyerhaeuser
Cosmopolis, WA, MC
Model mill calculations
Softwood HC, Sweden
Softwood, MC
Sweden
Hardwood, HC
Sweden
Hardwood, MC
Sweden
Size
500 tpd
500 tpd
1000 tpd
1000 tpd
n.a.
n.a.
680 tpd
600 tpd
400 tpd
1320 tpd
n.a.
n.a
n.a.
n.a.
Capital Cost (Smillions)
$8.8
$9-11
$14-16
$13-16
$13.5
$19.5
$8.0
$11.5
$9.5
$37.7
(installed)
$11-14.5
$6.5-$8.1
$11-14.5
$6.5-$8.1
Sources: As indicated in table.
Page 4-43
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Pollution Prevention m Pulp & Paper Section Four - Pulping
plant, while the vendor installs the equipment and in some cases operates the plant as well. The
technologies for onsite generation are much smaller in scale and produce slightly lower purity oxygen
compared to cryogenic methods, although this does not appear to affect pulp properties or mill operations
to any significant degree (van Lierop and Brown, 1987). Advances in pressure swing adsorption (PSA)
and vacuum swing adsorption (VSA) technologies have enhanced the attractiveness of onsite generation.
In one comparison, capital costs for a PSA oxygen system ranged from $220,000 to $525,000 and
produced oxygen at a cost of $50 to $75 per ton. This compares to prices of $100 per ton for liquid
oxygen (Bansal, 1987), although costs for liquid oxygen depend closely on transportation distances and
could be lower near major population centers. Onsite oxygen plants are available from numerous
industrial gas vendors such as Liquid Air, Air Products, Airco, and Union Carbide. The cost advantage
of onsite generation will depend on the volume demand and the proximity of the mill to sources of liquid
oxygen. Onsite production becomes attractive once demand reaches around 10 tons per day. Other areas
of the mill that may use oxygen include oxygen-reinforced extraction stages, wastewater treatment, and
black liquor oxidation. Potential oxygen requirements for a 1,000 tpd mill (as reported by an equipment
vendor) could reach as much as 110 to 150 tons per day, depending on the number of oxygen applications.
These figures are shown in Table 4-10.
Data from Idner (1988) indicates that, based on Swedish experience, there are substantial savings
in variable costs associated with oxygen and that these tend to offset higher capital costs. Table 4-11
suggests that variable costs would be lowered by 21 to 56 SEK ($3.40 to $9.08) per ton, depending on
the wood species (hardwood or softwood) and consistency of the pulp. When annualized capital costs are
added to operating and maintenance costs, the change in annual costs compared to conventional bleaching
range from a decrease of 16 to 23 SEK ($2.59 to $3.73) per ton for MC softwood pulping to an increase
of 25 to 41 SEK ($4.05 to $6.65) per ton for HC hardwood pulping. These figures assume no
requirements for changes in recovery system capacity, and credits for avoided investment in C- or D-stage
equipment are also not included.
At other mills considerably greater investment may be required, particularly to upgrade pulp
washing equipment. Successful implementation of oxygen delignification requires effective pulp washing
both in front of and following the oxygen stage. Inadequate washing of brownstock pulp prior to oxygen
delignification will result in increased oxygen consumption and possibly excess heat generation in the
reactor. Post-oxygen washing must also be thorough in order to recover as much dissolved lignin as
Page 4-44
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-10
Potential Oxygen Demand at Bleached Kraft Mill
Area of Mill
Oxygen delignification
White liquor oxidation
Oxidative extraction
Black liquor oxidation
Lime kiln enrichment
Wastewater treatment
TOTAL [a]
Potential Oxygen Demand (tpd)
25
5
5
60
1-5
15-50
111-150
[a] The use of oxygen for black liquor oxidation is limited to mills with older recovery
boilers. Oxygen use in wastewater treatment is predicated on the existence of an oxygen-
activated sludge treatment system, which not all mills have. Thus, for most mills, oxygen
demands will be considerably lower.
Source: Deal et al. (1991).
Page 4-45
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-11
Change in Total Costs for Oxygen Bleaching vs.
Conventional Bleaching in a 600 mt/d Swedish Mill
(SEK/metric ton)
Investment
(SEK millions)
Softwood (Pine)
HC
70 to 90
MC
40 to 50
Hardwood (Birch)
HC
70 to 90
MC
40 to 50
Variable costs
Maintenance'31
Capital Costsw
TOTAL
-56
10 to 13
44 to 57
-2 to 14
-54
6 to 7
25 to 31
-23 to -16
-29
10 to 13
44 to 57
25 to 41
-21
6 to 7
25 to 31
10 to 13
Notes: Assumes no changes in recovery system required.
$1.00 U.S. ~ 6.17 SEK (1988)
MC = medium consistency
HC = high consistency
[a) 3% of the investment
w Depreciation 15 years and 10% interest
Source: Idner (1988).
Page 4^6
-------
Section Four - Pulping Pollution Prevention in Pulp & Paper
possible before chlorioation. For mills that require a considerable upgrade in pulp washing equipment,
the cost of conversion may be closer to $15 million
433 Pollution Prevention Potential
From an environmental standpoint, oxygen delignification offers two distinct advantages. First,
by continuing the delignification process begun during kraft cooking, it reduces the amount of lignin
carried through to the chlorination stages. Reductions in the amount of lignin entering the bleach plant
translate directly into reductions in most pollutants of concern, i.e., BOD5, color, and organochlorines
(Tench and Harper, 1991). Table 4-12 shows the impacts of adopting oxygen dehgnification on pollutant
parameters such as BOD5, COD, TOC1, and acute fish toxicity. BOD5 declines by approximately 32
percent, COD by 43 percent, and TOC1 by 50 percent.
Second, the effluent from the oxygen stage is recycled through the pulp mill recovery cycle, hi
a conventional bleach plant, these solids would only be removed following chlorination. Such post-
chlorination effluents cannot be recovered due to their corrosiveness to process equipment and must go
to wastewater treatment (see Figure 4-12). Therefore, when coupled with increased recycle or close-up
of the effluent stream, oxygen pre-bleaching normally reduces effluent flows (and associated sludge
volumes) from the mill as well.
Oxygen also puts the mill on the track towards "zero effluent" pulping and chlorine-free bleaching.
Should regulations or market forces require further reductions or elimination of chlorine compounds, mills
that have previously installed oxygen delignification will be able to respond more rapidly.
43.4 Compatibility With Downstream Bleaching Stages
Oxygen delignification is compatible with most conventional bleaching sequences, hi addition,
since oxygen delignification has the potential to lower the pre-bleach stage kappa number by as much as
50 percent (and up to 65 percent for the newer two-stage systems), a variety of innovative bleaching
methods may also be applied, hi a typical application, the mill follows the oxygen stage with a mixture
Page 4-47
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-12
Pollutant Impacts of Oxygen Delignification
Versus Conventional Pulping/Bleaching
Parameter
Kappa no.
BOD7, kg/mt
COD, kg/mt
TOC1, kg/mt
Reduction in acute
toxicity to fish, % of
reference
Softwood (Pine)
Conven
-tional
32
14
80
5 to 5.5
-
HC
18
10.5
50
3 to 3.5
50 to 60
MC
15
9.5
45
2.5 to 3
60 to 70
Hardwood (Birch)
Conven
-tional
20
14.5
50
2 to 2.5
~
HC
14
11.5
40
1.5 to 2
n.a.
MC
12
10
35
1.5
n.a.
n.a. = not available.
MC = medium consistency
HC = high consistency
Source: Idner (1988).
Page 4-48
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
Pulp Mill
Bleach Plant
C/D E D E D
Alkaline sewer <
Primary
Treatment
>Acid sewer
Secondary
Treatment
Figure 4-12. Illustration of typical wastewater flows at bleached kraft mill.
Final
effluent
Source: Eastern Research Group, Inc.
Page 4-49
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. n i e D Section Four - Pulping
Pollution Prevention in Pulp & Paper ,
of chlorine and chlorine dioxide, followed by a caustic extraction stage, another C1O2 bleaching and
extraction stage, and a final bleaching stage, i.e. OCuEDED.
A significant benefit of oxygen delignification is that it will allow the mill to increase the
substitution rate of chlorine dioxide for chlorine without requiring additional chlorine dioxide generating
capacity. A typical mill operating currently at 30 percent C1O2 substitution, for example, could operate
at 70 percent in one of two ways: (1) increase its chlorine dioxide generating capacity, or (2) add an
oxygen delignification stage. Under the second option, the mill can cut back on the application of
chlorine, thereby increasing the ratio of C1O2 to C12 (the substitution rate) using its existing chlorine
dioxide capacity. Drawing from a database of actual installations, Brunner and Pulliam (1992) have
recently presented data for a model 1,000 tpd mill that indicated the latter option results in both lower
capital and operating costs.
Other possibilities include "short sequence" bleaching, in which oxygen delignification is followed
by three rather than five bleaching stages (i.e., OCoED or ODEOPD versus OCEDED). While short
sequence bleaching has the benefits of lower capital costs and, if carefully engineered, reduced effluent
flow, its feasibility for an individual mill will depend on the pulp quality requirements, including the target
brightness level. With fewer bleaching stages there is less opportunity for "fine timing" the pulp
characteristics and probably more variability in pulp quality. Integrated mills are more likely to consider
short-sequence bleaching than market pulp producers. Some conventional CEDED mills have succeeded
in raising capacity by a substantial amount by installing oxygen delignification and then splitting their
single 5-stage line into two 3-stage lines.
Finally, when combined with extended delignification, mills may be able to produce oxygen
delignified pulps with kappa numbers below 8 to 10, although minor losses in strength or yield may occur.
These pulps are extremely bleachable and may be bleached to 80+ brightness using small amounts of
chlorine dioxide (ECF bleaching), to 70+ using one or more peroxide-based stages (e.g., Lignox), or
potentially to 85+ with ozone and peroxide (OZEP), depending on tree species.12
12 Hardwoods are easier to bleach than softwoods.
Page 4-50
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Section Four - Pulping Pollution Prevention in Pulp & Paper
43.5 Impacts on Other Aspects of Mill Operations
Recovery Boiler Operations
The main obstacle to more widespread adoption of oxygen delignification is likely to be the
impact of an increased black liquor solids load on the recovery boiler. With the addition of a pre-
chlorination oxygen delignification stage, the amount of solids recovered per ton of pulp that are
subsequently directed to the recovery boiler system will increase. According to one analysis (Tench and
Harper, 1987), the recovery of 80 percent of the oxygen stage solids will generally increase the solids
going to the boiler by 50 to 55 kg per metric ton (approximately 3 percent) for hardwoods and by 30 to
35 kg per metric ton (approximately 2 percent) for softwoods. Additional post-oxygen washing stages
can increase this further still. In particular, at older mills where pre-chlorination washing is currently
inefficient the increase could be as much as 10 percent.
While some mills may operate with surplus boiler capacity, in other cases boiler throughput may
represent a production bottleneck. The burning of additional solids beyond boiler design capacity will
result in ash being built up on the boiler heat transfer surfaces, necessitating more frequent shutdowns to
permit the boiler tubes to be washed out (Clement, 1993). Options for accommodating a higher level of
solids per ton of pulp are available, however, and were reviewed in Section 4.2.5.
Air and Solid Waste Emissions
As with extended delignification, the introduction of an oxygen stage shifts some of the
delignification load ahead of the bleach plant, permitting recovery of additional solids from the effluent.
The processing of this additional effluent in the kraft recovery system may produce slight increases in air
emissions (from the recovery furnace) and solid wastes such as ash and precipitates (from the recovery
boiler), dregs (from the dregs washer) and grits (from the lime slaker).
The modern recovery furnace is equipped with sophisticated air emissions control equipment such
as electrostatic precipitation (ESP). Precipitates (primarily sodium sulfate and sodium carbonate) are
returned to the liquor makeup system, hence the only material losses are the ash, dregs and grits. These
are usually landfilled. The introduction of an oxygen stage and subsequent recycle of oxygen filtrate will
Page 4-51
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Pollution Prevention in Pulp & Paper . Section Four - Pulping
probably result in a minor (i.e., less than 5 percent) increase in the quantity of these materials going to
the landfill.
Energy Requirements
In substituting oxygen for chlorine, some changes in direct and indirect energy consumption will
result. Chlorine is manufactured offsite in an energy-intensive process that consumes approximately eight
times the amount of energy per unit of bleaching power compared to oxygen (McDonough, 1986).
Oxygen may be produced offsite and transported to the mill, in which case the energy costs will be
reflected in the delivered price, or manufactured onsite using energy supplied by the mill. In the latter
case, the energy cost may be ignored by the mill since many mills self-generate 100 percent of their
energy requirements through incineration of wood and pulping byproducts. The true cost of further energy
requirements may depend, therefore, on whether energy can be sold offsite into the local power grid.
Table 4-13 presents information on changes in energy requirements for a mill adopting high consistency
oxygen delignification. Site-specific factors could result in substantial variations from these figures.
4.4 OZONE DELIGNIFICATION
Ozone (O3) is an extremely powerful oxidant whose potential as a pulp bleaching agent has been
recognized for some time. Until recently, the integration of ozone into the bleach line has been limited
because of the detrimental effects it has had on pulp quality and strength properties. The successful
"taming" of ozone has been greatly anticipated in the industry, since it opens the door to elimination of
chlorine compounds in bleaching and raises the possibility of complete closure of the mill's bleach plant
and ultimately the total mill's effluent cycle. As a result of a considerable research effort, the past year
has seen the startup of the first two full-scale mills (1,000 tpd or more) operating ozone bleaching lines.13
Ozone bleaching equipment is now being offered by all of the established vendors of pulp bleaching
equipment.
13 To date, only limited details concerning these startups are available.
Page 4-52
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Section Four Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-13
Change in Energy Consumption for HC Oxygen Delignification
Using Softwood Kraft at 50% Oxygen Delignification
Energy Type
Steam usage,
kg/metric ton
Power usage,
kWh/metric ton
Source
Bleach plant
Chemical preparation
Evaporator, increase
Recovery boiler, increase
TOTAL
Bleach plant and chemical
preparation
Turbine increase
TOTAL
Kiln fuel, increase in liters oil/metric ton
C^DED
500
370
0
0
870
160
0
160
-
OCoEoD
600
250
40
-100
790
170
-10
160
2
Source: Tench and Harper (1987).
Page 4-53
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Pollution Prevention m Pulp & Paper Section Four - Pulping
The movement towards ozone bleaching has been achieved through advances in determining the
optimum conditions for ozone bleaching, developments in ozone generation equipment, and the relative
economics of ozone use. The key process conditions for ozone bleaching have been summarized by
Shackford (1992):
Pulp pH The pH of the reaction is the most important variable. At pH greater than 3, ozone
becomes less selective towards pulp and greater amounts of ozone will be consumed;
Pulp washing - The carryover of organic materials from brownstock or post-oxygen washing must
be minimized (below 10 kg COD per ton pulp);
Reaction temperature - Ambient temperatures (around 20 °C) appear to be optimal for an ozone
stage. At higher temperatures there will be increased ozone consumption for a given level of
delignification;
Metals buildup - The ozone stage operates in an acid medium, and is generally situated between
two alkaline stages. In this configuration, the ozone stage can become a "trap" for metal ions that
are present in wood pulp. These metals will catalyze the decomposition of ozone, thereby
releasing hydroxy radicals, which are destructive to pulp. To reduce metals buildup, a small purge
of acid filtrates upstream of the ozone stage is probably necessary.
Reaction mechanisms and process economics will determine the best consistency for the system. Low,
medium, and high consistency bleaching each have their own advantages and disadvantages:
• At low consistency, the stock is more homogenous and the reaction with ozone tends to be
more uniform. The larger volumes of water reacted with the pulp, however, means larger-scale
equipment;
• Advantages of "gas phase" or high consistency systems (35 to 45 percent) include potentially
lower ozone consumption, the ability to operate the reaction at or near atmospheric pressure,
greater ease of control of reaction temperature, and better water balance for recovery of all
effluents. Disadvantages include the need for additional presses to dewater the pulp and the
greater potential for pulp degradation (and hence lower yield). Note, however, that with a high-
consistency oxygen stage prior to the ozone stage the mill can forego the investment in pulp
pressing equipment;
• At medium consistency (12 to 14 percent) there may be an increase in ozone and acid
consumption compared to a gas phase system, but pulp handling equipment is less complex.
Ozone delignification is performed using techniques and equipment similar to that used in oxygen
delignification (see Figure 4-13). Peak ozone delignification efficiency has been found to occur at pH 1.0
to 2.0, thus pulp is normally treated with sulfuric acid prior to ozonation. In a high consistency system,
Page 4-54
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
Press
Pump
f=.
^•B
/C
nc
rt
r
n
LI
TT
J_L
;y
-- ^
ftH
pi i i
rtrlJ 1
2
A
^^
Flutter
Discharger
^^ Dilution
Diluted Stock
Out
Figure 4-13. Equipment for high-cxjnsistency (HC) ozone delignification.
Source: Miller, 1992.
Page 4-55
-------
Pollution Prevention m Pulp & Paper Section Four - Pulping
as shown, the acidified pulp is fluffed and deposited in the ozone reactor. Ozone gas generated onsite is
delivered to the pulp in an oxygen carrier gas. During reaction, the ozone is consumed and the carrier
gas is recovered and either returned to the ozone generator or used elsewhere in the mill (in an oxygen-
reinforced extraction stage or in the wastewater treatment plant, for example). The equipment for ozone
generation is a crucial part of the process and accounts for a high percentage of the costs. Ozone
generation is discussed in the section below.
Ozone Generating Equipment
The technology for large-scale ozone generation has been used for many years in industrial
applications such as bleaching of textiles, bleaching of clay in the manufacture of paper fillers, and in
municipal drinking water systems. Its use in pulp bleaching, therefore, represents only a new application
of an established technology. Major suppliers of ozone generating equipment and onsite ozone suppliers
include:
Ozone Generating Equipment Onsite/Over-the-Fence Ozone Supply
Ozonia Liquid Air
Emery Trailgaz Air Products
Capital Controls Praxair (formerly Linde)
Sumitumo MG Industries
Ozone generation requires large amounts of power, about 8 kWh per kilogram of ozone (Singh,
1982).14 Ozone is most commonly generated from oxygen or oxygen-containing gases (e.g., air) using
the corona discharge method. The generating equipment consists of a series of tubes through which the
feed oxygen or air flows. As high voltage is applied across the discharge gap, free electrons in the corona
collide with the diatomic oxygen and cause disassociation of the O2 molecules, which recombine to form
ozone. Ozone is unstable and will decompose to molecular oxygen, thus ozone must be generated onsite
and fed immediately to the pulp reactor.
14 While chlorine dioxide generation requires approximately the same amount of energy as ozone,
ozone is theoretically twice as powerful a bleaching agent.
Page 4-56
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Section Four - Pulping Pollution Prevention in Pulp & Paper
The ozone reaction with pulp is exothermic, and a large amount of the energy required for
production (up to 90 percent) is converted to heat. This heat must be dissipated, normally through the
application of cooling water, and may be recovered.15 Cooling water requirements sufficient to keep the
gas temperature stable and prevent ozone decomposition are in the range of 2,500 to 4,000 liters per kg
of ozone.
Ozone can be generated from air, although the low concentration of oxygen in air make this
commercially unfeasible. Instead, it is normally produced from oxygen. Recent articles suggest that most
mills will purchase liquid oxygen (the purest form) for conversion to ozone, although it is very likely that
ozone generation systems will also be coupled with on-site oxygen plants (Byrd and Knoernschild, 1992).
Commercial ozone generators have a capacity of between 450 and 1,400 kg of ozone per day, and most
mills will require several large-sized generators running in parallel.16
Since only a small percentage of the oxygen feed gas is converted to ozone, there is a substantial
incentive to recycle the carrier gas back into the ozone generator. These off-gases pick up volatile
components as a result of their contact with the pulp, however, and must be first passed through a
purification system that combines wet scrubbers, catalytic converters, and desiccant dryers. Some work
is being done on systems that separate the ozone from the oxygen gas prior to pulp contact. This would
allow the oxygen carrier gas to be recycled directly to the ozone generator (Byrd and Knoernschild, 1992).
4.4.1 Number of Installations
Table 4-14 shows the history of ozone pilot plant and full-scale installations worldwide. Early
work in the U.S. was done at the Scott Paper mill in Muskegon, MI and then at Weyerhaueser's Longview
mill in Washington State. PAPRICAN was also heavily involved in early research work at their Pointe
Claire, Quebec headquarters. In 1989, Union Camp (Wayne, New Jersey) installed a 25 tpd ozone system
at its mill in Eastover, South Carolina. This $6 million experimental project has provided promising
15 Normally, this low level heat cannot be used in the mill and will be dissipated using a cooling
tower.
16 The Union-Camp installation described in the next section operates five ozone generators.
Page 4-57
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-14
Ozone Pilot and Full-Scale Plants Worldwide
Year
1971
1973
1975
1976
1982
1982
1988
1989
1990
1990
1991
1991
1991
1991
1991
1992
1992
1992
Location
PAPRICAN
Scott Paper
PAPRICAN
CTP
Myrens Verksted
Weyerhaueser
PWA
Union Camp
Wagner-Biro AG
Kraftanlagen
Heidelberg
Lenzing AO
OZF
E.B. Eddy Forest
Products
PAPRICAN
CTP
Lenzmg AG*
Union Camp*
Sodra*
Pointe Claire, Quebec
Musckegon, MI
Pointe Claire, Quebec
Grenoble, France
Hofmen-Hellefos, Norway
Longview, WA
Stockstadt, Germany
Eastover, SC
Graz, Austria
Beienfurt, Germany
Lenzing, Austria
Cratkorn, Austria
Espanola, Ontario
Pointe Claire, Quebec
Grenoble, France
Lenziug, Austria
Franklin, VA
Monsteras, Sweden
Capacity
(tpd)
10
15
10
0.5
5
20
3
25
1
5
100
15
5
5
3
400
1000
1000
Consist-
ency
HC
HC
HC
HC
HC
LC
HC
HC
LC
HC
MC
LC/HC
LC/MC/
HC
MC
MC/HC
MC
HC
MC
Bleaching sequences
Z, (PZ)
Z
Z
Z
Z
OZD, OZDED
Z
OZEDED
OZP
OZEP
(EOP)ZP
any sequence
0(2)...
OZEP, OZED
0(pZE)P
(EOP)ZP
OZEoD
OZEoP (potential)
OZEP
Pulp type
Mechanical
Hardwood kraft
Sulfite, kraft
Mechanical
Mechanical, sulfite
Softwood kraft
Sulfite
Kraft
Sulfite, kraft,
nonwood fiber
Mg bisulfite
Hardwood,
dissolving
Kraft, sulfite
Softwood kraft,
hardwood kraft
All types
All types
Hardwood,
dissolving
Kraft, integrated
Market kraft,
HW/SW (HW only
so far)
* Full-scale installation.
Source: Liebergott et al., 1992.
Page 4-58
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Section Four - Pulping Pollution Prevention in Pulp & Paper
results over a four-year period. Based on experiences at this mill, the company just recently (September
1992) completed the startup of a full-scale ozone bleaching line at its 1,000 tpd mill in Franklin, Virginia,
the first in the world to operate at that scale. The mill uses oxygen, ozone, and chlorine dioxide to
produce elemental chlorine-free (ECF) pulp from southern pine. All of the pulp will be used for onsite
production of bleached uncoated free sheet and coated and uncoated bleached board. Target brightness
is 83 to 85 GE, and the bleaching sequence is OZE0D (Nutt et al., 1992). Visits to the Franklin mill by
EPA contractors have confirmed that there have been no major operational difficulties associated with the
ozone bleaching process to date.
Among the commercial-scale ozone projects in the startup or planning stages (as of 1992) are the
following:
• The Monsteras mill in Sweden started up a 1,000 tpd medium consistency ozone bleach line
at approximately the same time as Union Camp's startup. The mill is reported to be using 30
kilos of peroxide per ton of pulp to produce totally chlorine-free market pulp (TCF) at 88 to 89
ISO brightness for sale in Germany. To date, the mill has pulped only hardwoods;
• The Lenzing mill in Austria has installed medium consistency ozone in an EOPZP configuration
at its 100 tpd dissolving pulp mill. The mill has recently converted an adjacent 400 tpd line;
• MoDo has purchased an ozone system from Kamyr for installation at their mill at Husum,
Sweden;
• SCA Wifsta-Ostrand in Sweden has licensed ozone technology from Union Camp for use at
their Timra kraft mill;
• The E.B. Eddy mill in Espanola, Ontario is considering installation of a full-scale ozone plant
following successful operation of Kamyr's pilot plant.
For the Franklin mill installation, Union Camp selected a high consistency gas phase ozone stage.
The advantages claimed for gas phase reaction are possibly lower ozone consumption (compared to
medium consistency), and the ability to operate at or near atmospheric pressures. In Union-Camp's case,
the selection of high consistency may also have been influenced by the fact that a high consistency oxygen
stage was already in place. This eliminated the need for installation of a high consistency pulp press prior
to the ozone stage. Through redesign of the ozone reactor, Union Camp claims to have solved the
problem of nonuniformity of reaction that had been experienced in previous HC lab and pilot plant trials
(Nutt et al., 1992).
Page 4-59
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. 0 , j D or Section Four - Pulping
Pollution Prevention in Pulp & Paper ' *
Union Camp has formed a worldwide marketing alliance with Sunds Defibrator of Sweden to
license its ozone bleaching technology in the pulp and paper industry under the name C-Free™ The
technology is based on use of oxygen delignification, gas phase ozone, and a small amount of chlorine
dioxide, which enables the licensee to produce pulp at full market brightness.
4.4.2 Costs and Economics
Capital costs for the ozone delignification equipment will depend on the type of system selected
(i.e., gas phase or medium consistency). This in turn is dependant on the charge of ozone required to
achieve target pulp properties (brightness and yield) and the economics of the individual mill. Mills using
ozone as a "bulk delignification" stage, (i.e., as the primary delignifying agent), may choose high
consistency due to the potential for reacting more ozone in a single stage. Others using ozone at more
moderate levels may favor medium consistency.
In comparison with chlorination stage bleaching, ozone requires additional process equipment
(pulp press, high shear mixers, acid handling system) but benefits from the ability to use cheaper
construction materials, as corrosion problems are less severe compared to chlorine bleaching. Depending
on how close the mill comes to closing its effluent cycle and whether it maintains one or more D stages,
there should be reduced costs for effluent treatment and bleach plant scrubbing systems (Deal, 1991).
Cost savings from these areas would be greater for a greenfield mill compared to a retrofit. The final cost
for the Franklin mill installation has been recently cited as $113 million (Ferguson, 1992b).
Bleaching costs for ozone, including ozone generation, are lower than for conventional sequences.
Power consumption for an oxygen feed ozone system producing 83 to 85 percent brightness pulp is about
8 to 9 kWh per kg O3 or $0.64 per kg based on a power value of $0.04 per kwH (Nutt et al., 1992). Fuel
values from additional solids recovery may provide further payback, depending on recovery capacity. Nutt
et al. (1992) compared operating costs of the OZE0D line at the Franklin mill with the mill's 1970s
era CEDED bleaching sequence, and against two more contemporary processes (with 100 percent chlorine
dioxide substitution) that might be found at numerous North American mills (costs are indexed with
CEDED and DEDED bleaching set equal to 100). Table 4-15 indicates that bleaching costs for the
OZE0D line will run 48 and 83 percent of the costs for CEDED bleaching on pine and hardwood,
Page 4-60
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-15
Bleaching Chemical Costs of Ozone Versus Conventional Sequences
at Union Camp's Franklin. Virginia Mill
Sequence
C-E-D-E-D
O-Z-EO-D
Relative Costs
Pine
100
48
Hardwood
100
83
D-E-D-E-D
O-D-E-D
O-Z-EO-D
100
56
32
100
73
57
Note: Bleaching chemical costs of ozone-based sequences are shown relative to those of
conventional sequences (costs equal to 100 for the conventional processes).
Assumptions for costing purposes are shown below.
Source: Nutt et al. (1992).
ASSUMPTIONS
Chlorine $153.00 per ton
Chlorine Dioxide $0.32 per Ib
Oxygen $53.00 per ton
Magnesium sulfate $485.00 per ton
Caustic co-purchased with chlorine on ECU basis. $215.00 per ton
Caustic purchased independent of chlorine $340.00 per ton
Cost of preparing oxidized white liquor $29.00 per ton as NaOH
Sulfuric acid $68.00 per ton
Chelant $0.51 per Ib
Ozone $0.29 per Ib
Includes byproduct saltcake credit
Used for CEDED requirements and used for Vi of
O(DC)ED requirements
Used for OZED requirements and used for '/2 of
O(DC)ED requirements
Based on power costs of $36/MwH and Union
Camp's design of a recirculating ozonegeneration
system
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. n , f D r Section Four - Pulping
Pollution Prevention in Pulp & Paper . i_Ł
respectively. When compared to a more contemporary DEDED line, OZE0D bleaching costs will run at
32 and 57 percent of the costs.
Union Camp's decision to retain a chlorine dioxide stage was based on the expectation that
elimination of elemental chlorine would be sufficient to guarantee satisfactory effluent levels. The use
of ozone in the bleach line, however, introduces the possibility of eliminating all chlorine-containing
compounds to produce totally chlorine-free (TCP) pulp. Hydrogen peroxide would be used as a final
brightening agent, replacing chlorine dioxide in a TCF sequence. Nutt et al. (1992) indicate that replacing
C1O2 with peroxide would raise operating costs back to the level of a CEDED process (e.g., by 52 percent
for softwood and 17 percent for hardwood) if 90 percent ISO brightness were required (peroxide costs
would be around $30 to 40 per ton).17 Lower brightness TCF pulps (low 80s) could likely be produced
at significantly lower costs (under $10 per ton premium) if there was greater market acceptance.
Since either oxygen delignification or extended cooking (or both) are considered prerequisites for
successful ozone bleaching, the more widespread adoption of these technologies may increase interest in
ozone. Also, ozone's cost vis-a-vis conventional bleaching sequences has improved as it is now more
likely to be used as a replacement for more expensive chlorine dioxide rather than less expensive chlorine.
4.4.3 Pollution Prevention Potential
Due to its powerful bleaching effect, ozone has the potential to replace most if not all of the
chlorine-based bleaching agents used in conventional pulp bleaching. At Union Camp, the bleaching
sequence has been simplified to OZE0D, eliminating all elemental chlorine and retaining just one stage
of chlorine dioxide bleaching. Emissions from the process are extremely low because of the ability to
recycle all of the O, Z, and E0 stage effluents. Table 4-16 summarizes the effluent quality of the new
OZE0D line at the Franklin mill under ozone bleaching. As seen, total organic halides are below 0.1 kg
per ton in effluent, chloroform is a very low 0.0015 kg per ton, BOD is below 2, COD is below 6, and
color is below 1.5 (traditional color levels are around 100 to 300 kg per ton). The mill has been unable
to detect dioxins using the most sensitive testing methods available, even after boosting chlorine dioxide
consumption in the final stage.
17 Note that the technical feasibility of producing 90 percent ISO softwood pulp using an OZEP
sequence is debated within the industry. Softwoods are generally more difficult to bleach than hardwoods.
Page 4-62
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-16
Emissions from Ozone Bleach Line at Union Camp's
Franklin, Virginia Mill w
Parameter
TOX, kg/ADT Pulp
Effluent
Chloroform, kg/ADT
BOD5 kg/ADT
COD, kg/ADT
Color, kg/ADT
Effluent Volume m3/ADT
gal/ADT
Pine
0.04
0.075
0.0015
2.0
6.0
1.5
7.5
1,800
Hardwood
0.03
0.06
0.0015
1.0
2.0
0.5
7.5
1,800
w Running an OZE0D bleaching sequence to produce 83 brightness pulp.
Source: Nutt et al. (1992).
Page 4-63
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. D , , r, ar Section Four - Pulping
Pollution Prevention in Pulp & Paper
Table 4-17 presents similar effluent data from the PAPRICAN ozone pilot plant running several
bleach sequences. Compared to a conventional C^DED sequence the OZE0D sequence resulted in the
following reductions: BOD (62 percent), COD (53 percent), Color (88 percent), and TOC1 (98 percent).
hi comparison with a more modern ODE0DED sequence, BOD and COD levels of the OZE0D sequence
were slightly higher (by 20 and 14 percent, respectively), while effluent color decreased by 48 percent and
TOC1 fell by 80 percent.
Among the numerous ozone-based bleaching sequences currently being investigated in labs and
pilot plants around the world are: OZEP, OZED, OZPY, OZEPY, ZO^PY, and OZEuPY. (Note: O^
refers to an oxygen stage using a wash of ozone stage effluent and Y is sodium hydrosulfite.) The
absence of elemental chlorine in these sequences and the elimination of all chlorine-based compounds in
some indicates that effluents from future mills using ozone will be extremely low in pollutants of current
concern.
4.4.4 Impacts on Other Aspects of Mill Operations
Pulp Quality
One concern raised by some in the industry is that ozone-bleached pulps tend to be of lower
strength and hence lower quality than those produced by conventional bleaching processes. Most of these
concerns center around observed decreases in the viscosity of pulps bleached using ozone. Viscosity has
traditionally been used as an indicator of pulp strength, and ozone-bleached pulps have in fact been found
to have lower viscosities than conventionally-bleached pulps of similar kappa number. As discussed by
Liebergott (1992b), however, numerous researchers have found that the viscosity-strength relationship is
different for non-conventionally bleached pulps, and that despite their lower viscosities, ozone-bleached
pulps maintain their strength properties.18 (The same is true to some extent for oxygen-delignified pulps.)
Table 4-18 shows Union-Camp data on properties of conventional, oxygen-delignified, and ozone-
delignified pulps. Although the viscosity of the pulps decrease with the use of low-chlorine bleaching
sequences, the pulp strength properties (tear, breaking length) are maintained. Viscosity falls from 17 to
11 for softwood and from 14 to 12 for hardwood when moving from CEDED bleaching to OZE0D, while
18 Viscosity itself is not a functional end-use parameter, but it is used as a surrogate for strength.
Page 4-64
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-17
Effluent Properties of Ozone Bleaching Sequences
Bleach Sequence
C^DED
OCpEoDED
ODE0DED
OZED
OZEoPY or
OZEZP
Effluent Outfall (kg/ton)
Color
228
75
52
27
28
BOD
31
18
10
12
13
COD
101
50
41
47
49
TOC1
7.7
4.4
0.5
0.1
Basis: softwood kraft, brownstock kappa 30.2, final brightness 90% ISO
Source: Shackford, 1992 (based on PAPRICAN lab effluent data).
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-18
Properties of Pulps Produced
Using Alternative Bleaching Sequences
Parameter Units
Breaking length km
Zero span breaking length km
Burst m3/cm3
Tear dm3
TAPPI viscosity cp
Bulk crnVg
TAPPI opacity 577 ran
TAPPI scattering coefficient 577 nm
CEDED
SW
7.5
13.8
53
111
17
1.63
74
260
HW
5.9
14.2
39
87
14
1.65
78
354
O(DC)ED
SW
7.2
12.4
62
119
13
1.64
70
242
HW
5.8
13.3
41
87
13
1.72
79
366
OZED
SW
7.0
12.5
59
124
11
1.66
72
254
HW
5.7
13.7
39
85
12
1.71
79
368
Experimental: 83 GE brightness pulp, Valley beater refined to 350 mL CSF. Nonstandard
handsheet method used of acid sized pulp, fines retained, semistrained drying to simulate
paper machine conditions.
Source: Nutt (1993).
Page 4-66
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Section Four - Pulping Pollution Prevention in Pulp & Paper
burst and tear parameters are comparable or superior. Thus, the ozone-bleached pulps are different than,
but not necessarily inferior to, pulps produced using conventional sequences.
It should be noted that ozone is a toxic gas that must be handled properly. Ozone generators are
equipped with sensors that will shut off power to the unit if any leaks are detected. Ozone production
stops as soon as the power is cut. Under pressure, ozone may also present explosion hazards. For this
reason, high consistency systems that operate at or near atmospheric pressures may be considered safer
than medium consistency systems that operate under pressure.
One distinct safety advantage of ozone over chlorine is that the ozone is generated onsite.
Chlorine is generally shipped to the mill in 100-ton tanker cars; this gas must then be transferred and
stored onsite. Since ozone is produced on demand there is no onsite storage, hence only the small
quantities contained in the pipeline (several kilograms) would pose a danger.
Recovery Boiler Operations
The main obstacle to more widespread adoption of oxygen delignification is likely to be the
impact of an increased black liquor solids load on the recovery boiler. With the addition of a pre-
chlorination oxygen delignification stage, the amount of solids recovered per ton of pulp that are
subsequently directed to the recovery boiler system will increase. According to one analysis (Tench and
Harper, 1987), the recovery of 80 percent of the oxygen stage solids will generally increase the solids
going to the boiler by 50 to 55 kg per metric ton (approximately 3 percent) for hardwoods and by 30 to
35 kg per metric ton (approximately 2 percent) for softwoods. Additional post-oxygen washing stages
can increase this further still. In particular, at older mills where pre-chlorination washing is currently
inefficient the increase could be as much as 10 percent.
While some mills may operate with surplus boiler capacity, in other cases boiler throughput may
represent a production bottleneck. The burning of additional solids beyond boiler design capacity will
result in ash being built up on the boiler heat transfer surfaces, necessitating more frequent shutdowns to
permit the boiler tubes to be washed out (Clement, 1993). Options for accommodating a higher level of
solids per ton of pulp are available, however, and were reviewed in Section 4.2.5.
Page 4-67
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Pollution Prevention in Pulp & Paper Section Four - Pulping
Air and Solid Waste Emissions
The addition of an ozone stage will have a significant impact the chemical recovery cycle due to
the greatly reduced need for conventional bleaching chemicals and the ability to recycle the Z-stage
effluents. Since the Z-stage operates without chlorine-based chemicals, all of the Z-stage effluents can
be recycled through the recovery boiler, permitting recovery of additional solids from the effluent.19 The
processing of this additional effluent in the kraft recovery system may produce slight increases in air
emissions (from the recovery furnace) and solid wastes such as ash and precipitates (from the recovery
boiler), dregs (from the dregs washer) and grits (from the lime slaker).
The modern recovery furnace is equipped with sophisticated air emissions control equipment such
as electrostatic precipitation (ESP). Precipitates (primarily sodium sulfate and sodium carbonate) are
returned to the liquor makeup system, hence the only material losses are the ash, dregs and grits. These
are usually landfilled. The introduction of an ozone stage and subsequent recycle of ozone filtrate will
probably result in a minor increase in the quantity of these materials going to the landfill.
Energy Requirements
As discussed above in Section 4.4, ozone generation is approximately as energy-intensive as
chlorine dioxide, yet ozone is about twice as powerful a bleaching agent. Energy consumed onsite in the
generation of ozone will replace energy used offsite to produce chlorine and sodium chlorate. Since most
mills generate all of their onsite power requirements from pulping byproducts, it is possible that surplus
energy exists for ozone generation, especially where the mill is unable to sell power back into the local
utility grid.
19 A small purge of acid filtrates upstream of the ozone stage is generally necessary to avoid buildup
of metals in the pulp. Metals naturally present in the wood can catalyze the hydrolysis of ozone to
produce the hydroxy ion, which is destructive to pulp (Shackford, 1992).
Page 4-68
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Section Four - Pulping Pollution Prevention in Pulp & Paper
4.5 ANTHRAQUINONE CATALYSIS
In the 1970s, the addition of anthraquinone (AQ) to the pulping liquor was discovered to speed
up the kraft pulping reaction and increase the yield, with no deleterious effects on pulp properties (Holton
and Chapman, 1977). The anthraquinone acts as a delignification catalyst, and can cause a combination
of the following impacts: (1) increased pulp yield of up to 2 percent; (2) reduced chemical requirements
by up to 10 percent; (3) increased pulp production, and (4) lower cooking temperature (and energy
requirements).
The use of small amounts of AQ (around 0.05 to 0.1 percent on cooking liquor) catalyzes or
accelerates the fragmentation of lignin, rendering it more vulnerable to attack and dissolution by the
cooking chemicals. Most of the reacted AQ is removed with the spent liquor and presents no difficulties
in the recovery system. AQ is not detectable in pulps subject to further chemical bleaching (Blain, 1992).
By improving yield and lowering black liquor solids generation, anthraquinone addition has the
potential to facilitate the use of extended cooking or oxygen delignification at some mills. This would
include mills experiencing unacceptable yield drops from extended cooking, or where extended cooking
or oxygen delignification increases the boiler load beyond capacity.
According to Blain (1992), mills will find AQ most attractive if they are in a situation to benefit
from the "multiplicative" effects of AQ pulping. This refers to the effectiveness of AQ in (1) reducing
the level of organic and inorganic solids, and (2) increasing yield per ton of fiber.
4.5.1 Number of Installations
AQ pulping is reported to be used extensively in Japan to improve yield from what are relatively
expensive wood sources (Blain, 1992), and in at least two mills in Canada. This same source indicates
significant current interest in AQ as a means of achieving extended delignification and overcoming boiler
capacity bottlenecks. At current U.S. pulp wood and AQ chemical cost levels, the use of AQ is not
attractive unless it solves some bottleneck problem, usually in the recovery boiler. The trade journal
Paper Age (1990) has reported that over 100 mills worldwide use anthraquinone.
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Pollution Prevention in Pulp & Paper Section Four - Pulping
The patent on use of anthraquinone in pulping, held by ICI, is due to expire in a couple of years.
According to some, several pulp and paper companies are considering sourcing AQ and becoming a
supplier to the industry. One company is reported to be already importing AQ from India for use in pulp
cooking.
4.5.2 Costs and Economics
The potential for anthraquinone to increase yield or reduce chemical requirements implies a
reduction in the load on the recovery boiler. Thus, anthraquinone catalysis offers another potential means
to buffer the impacts of oxygen delignification or extended cooking on the recovery system. Holton and
Blain (1983) have examined the potential impacts of anthraquinone and concluded that its use could
compensate for an increase in chemical recovery load up to 7 to 8 percent. Table 4-19 shows that a 0.04
percent AQ charge on wood at a 1,000 ton per day mill could have the following impacts:
(1) raise yield by 0.75 percent (or an additional 7.5 tons per day);
(2) increase net costs by $5,262 per day or $5.15 per ton; and
(3) reduce boiler load 3.0 percent.
In some mills, this reduction in boiler load will be sufficient to enable the mill to accommodate
the additional solids load that results from oxygen delignification. The additional cost of AQ would, of
course, offset the cost savings that would otherwise result from using oxygen.
4.5.3 Pollution Prevention Potential
Based on the its potential for decreasing chemical requirements or facilitating adoption of oxygen
delignification, anthraquinone catalysis will reduce the formation and release of chlorinated organics. As
with other delignification modifications that lower the pre-chlorination kappa number, chlorinated organics
formation should decrease in approximate proportion to the drop in lignin content of the brownstock pulp.
Page 4-70
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-19
Costs of Anthraquinine Treatment
Parameter
Yield %
Wood input ODT/day
% on
Effective alkali charge wood
Price of AQ $/kg
Cost of softwood at mill $/ODT
Organics to black liquor ODT/day
Actual chemical charge ODT/day
Black liquor exit digester ODT/day
Black liquor solids: pulp kg/tonne
Boiler load reduction (organic) %
AQ charge kg/day
Cost of AQ charged $/day
Saving on wood $/day
Evap. steam @ $9/tonne $/day
Saving on evap. steam $/day
Lime kiln fuel $/day
Saving on lime kiln fuel $/day
NaOH released for ox. delig. kg/tonne
Net cost of AQ $/day
Net additional cost
($/ton bleached pulp) $/ADT
Anthraquinone Charge
(% on wood)
0.00%
48.00
2,083
14.50
11.00
140
1,062
569
1,631
1,631
0.0
0
0
0
18,000
0
10,000
0
0
0
0
0.03%
48.56
2,059
14.13
11.00
140
1,038
548
1,586
1,586
2.2
618
6,796
3,364
17,792
208
9,629
371
12.4
3,971
3.88
0.04%
48.75
2,051
14.00
11.00
140
1,030
541
1,571
1,571
3.0
821
9,026
4,487
17,723
277
9,507
493
16.5
5,262
5.15
0.06%
49.13
2,036
13.75
11.00
140
1,015
527
1,542
1,542
4.4
1,221
13,435
6,679
17,588
412
9,266
734
24.5
7,833
7.66
0.08%
49.45
2,022
13.50
11.00
140
1,002
514
1,516
1,516
5.6
1,618
17,796
8,552
17,472
528
9,037
963
32.2
10,365
10.14
Note: Under each option it is assumed there is no net change in pulp output. Costs shown are in
1992 Canadian dollars.
Source: Ontario Ministry of the Environment, (1992); calculated from data presented in Holton
(1983) and Bonsor (1988).
Page 4-71
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Pollution Prevention in Pulp & Paper Section Four - Pulping
4.5.4 Impacts on Other Aspects of Mill Operations
Recovery Boiler Operations
Depending on how it is used, anthraquinone could alternatively decrease, have no impact on, or
increase the recovery boiler load. One strategy is to use AQ to produce the same quantity of pulp using
less fiber. In this case, the solids load would decline. A more common application, however, is to use
AQ to facilitate implementation of extended delignification without the increase in solids load that
otherwise would occur (see Section 4.2). Finally, where maximum output from available fiber is the goal,
the mill could use AQ to boost yield such that additional solids are produced.
Air and Solid Waste Emissions
To the extent that AQ is used in such a way that it results in an increased solids load, air
emissions from the recovery boiler and solid waste byproducts from the recovery cycle may increase
slightly.
Energy Requirements
The impacts of AQ on energy requirements are expected to be negligible.
4.5.4 Environmental Effects
Anthraquinone is produced from coal tar generated in the coking process at steel mills. The
chemical is used in several industries, including textiles where it serves as an intermediary in dyestuff
manufacture. Most anthraquinone used in pulping is likely destroyed during the recovery process (i.e.,
incinerated). To our knowledge, no negative environmental effects have been reported in the literature.
Page 4-72
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Section Four - Pulping Pollution Prevention in Pulp & Paper
4.6 BLACK LIQUOR SPILL CONTROL AND PREVENTION
Accidental losses of black liquor may occasionally occur due to equipment failure, design flaws,
or human error. Losses can result from overflows or leaks from process equipment (spills), or as a
consequence of deliberate operator action (dumps) taken to avoid much mo^e serious consequences.
Equipment modifications can result in fewer spills or mitigate spill impacts, while spill prevention
programs can potentially have even greater impacts.
Losses of up to 5 percent of liquor volume were not uncommon at some mills 30 years ago. In
the absence of warning systems or detection procedures, many spills can go unnoticed for extended
periods of time. As more of the mill effluent is recovered through the application of measures such as
extended delignification techniques, accidental losses will comprise an increasing proportion of effluent,
hence efforts to minimize spills and their impacts are important.
Where no spill recovery system is in place, losses of black liquor will flow through the sewer to
the wastewater treatment system. Depending on the volume of the spill, the high level of BOD and COD
in the black liquor can result in a shock to the microbial action of the system, throwing it off balance and
degrading the quality of treated effluent. Production interruptions may be necessary to allow time for
the treatment system to return to equilibrium. A small but environmentally significant proportion of the
black liquor solids are also non-biodegradable, so if spilled to sewer, will be discharged into the receiving
waters. Spills are also a significant source of air emissions.
Effective loss control depends on a combination of good design, engineering, and operator
training. Design changes that can be implemented include: 1) physical isolation of individual pieces of
equipment so that spills can be collected and recovered, 2) modifications to the general floor drainage
system so that spills are collected and returned to the appropriate section of the recovery system, 3)
provision of additional backup storage capacity, 4) sensors and other systems that provide immediate
warning of potential or actual spill conditions, and 5) replacement of open-stage washing and/or screening
equipment with closed equipment.
An effective spill control system design is shown in Figure 4-14. The system would include
conductivity and pH probes in the process sewer to detect and identify the spill. Once detected, the spill
can be diverted to either a spill lagoon (in the case of weak spills that would not overload the treatment
Page 4-73
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
Process Sewers
Detection and Diversion
Concentrated
Spill
Spill
Tank
Weak
Spill
Normal Conditions
Weak Black
Liquor Storage
, Spill
Lagoon
As Effluent Treatment
Facility Loadings Allow
Effluent Treatment
Figure 4-14. Spill control system flow design.
Source: Edde, 1984.
Page 4-74
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Section Four - Pulping Pollution Prevention in Pulp & Paper
plant) or to a spill tank, where the spill would be held until it could be reintroduced into the recovery
system.
Operator training and awareness is equally important to prevention. Control of spills requires an
appreciation of the overall process, knowledge about locations where spills are likely to occur, and an
understanding that spill control and environmental protection is part of every employee's job.
4.6.1 Number of Installations
Improved spill control is now recognized as an essential element in the overall environmental
control of a modern pulp and paper mill. Newer mills are likely to have advanced spill warning and
control systems designed into the plant .from the start, and to use newer equipment design that is less
prone to spillage. In an older mill, the retrofitting of floor drains and other spill collection systems can
be quite difficult and very expensive, but some degree of improvement over the practices of the 1960s is
often possible. Instrumentation to warn of spills and facilitate rapid implementation of corrective measures
can normally be retrofitted relatively easily.
4.6.2 Costs and Economics
Costs for improving spill control are entirely site-specific, as they will depend more on the
physical layout of the -plant than on the particular process in use. No cost information has been collected.
Capital costs are more likely to be in the range of hundreds of thousands rather than millions, while
operating costs are low and will be partially or totally offset by recovered chemical and/or heat value,
provided the spill is routed to the recovery cycle.
4.6.3 Pollution Prevention Potential
Raw black liquor contains high levels of BOD, COD, as well as some persistent non-chlorinated
organics. As increased recycling of mill effluents is achieved through use of extended delignification and
oxygen, spills will account for an increasing percentage of total mill effluent.
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Pollution Prevention in Pulp & Paper Section Four - Pulping
4.6.4 Impacts on Other Aspects of Mill Operations
When spills are not recovered the heat and chemicals value of the black liquor solids is lost. A
high loss rate due to spills could result in a significant economic penalty in terms of lost heat and
chemicals.
4.7 ENZYME TREATMENT OF PULP
The potential for using cultured enzymes to assist in pulp bleaching has gained significant
attention in the past few years. Research in the field of biotechnology has isolated specific, naturally-
occurring microorganisms that produce enzymes capable of weakening the lignin bonds in pulp fibers,
thereby providing a boost to delignification. Experimentation with these enzymes has accelerated recently
and it is believed that numerous mills in North America and Scandinavia have been and are now running
full-scale trials, some of which are sufficiently long to be considered as production campaigns.
The enzymes of most interest in pulp bleaching are the xylanases, which are secreted by wood-
inhabiting microbes. There are numerous theories concerning how xylanases catalyze the hydrolysis of
xylan, the main bonding agent between lignin and cellulose. This action is believed to improve the
accessibility of bleaching chemicals to the pulp and enhance the extractability of the solubilized lignin.
Figure 4-15 shows the hypothesized reaction mechanism for xylanases. The xylan molecule "clips" the
bonds of xylan within the long chain of the xylan molecule. This clipping is believed to disrupt the
bonding between lignin and cellulose in pulps, thereby permitting easier attack of lignin by the pulping
chemicals.
Xylanase application in the mill has proved to be relatively simple. Conditions favorable for xylan
reaction with pulp include:
• pH between 4 and 6;
• temperatures of 40 to 55 °C;
• reaction times of between 30 and 180 minutes; and
• pulp consistency between 2.5 and 12 percent.
(Source: Farrell, 1992).
Page 4-76
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
Figure 4-15. Hypothesized reaction of xylanase with pulp.
Figure 4-16. Equipment configuration for xylanase application.
Source: Senior and Hamilton, 1992.
Page 4-77
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Pollution Prevention in Pulp & Paper _ Section Four - Pulping
These conditions can be easily obtained by mixing sulfuric acid and xylanases with pulp coining off the
brownstock washer as it enters the high density storage chest, in effect using the brownstock storage as
a reaction vessel (see Figure 4-16). According to Senior and Hamilton (1992), the addition of a post-
xylanase wash stage can significantly enhance the biobleaching effect, but this is not common practice.
Xylanase reaction appears to have no discernible impact on pulp quality or yield. In mill trials,
vendors report reductions in active chlorine requirements of between 15 and 50 percent (Senior and
Hamilton, 1992). Informal contacts indicate that actual reductions are nearer to the low end of these
estimates. Figure 4-17 shows the lower AOX levels associated with xylan-treated pulps versus control
pulps in a 20 percent chlorine dioxide substitution bleaching sequence. Some improvement in brightness
ceilings have also been observed, as shown in Figure 4-18. The reduction in demand for bleaching
chemicals can be used by mills in a variety of situations, including boosting of brightness levels, higher
substitution of chlorine dioxide, or to facilitate production of totally chlorine-free pulp (as at Aanekowski,
Finland).
Xylanases suitable for use in mill bleaching trials are available from several biotechnology and
chemical concerns around the world, including: Genencor International, logen, Novo Nordisk, Sandoz, ICI
Canada, and Voest-Alpine. Xylanase application rates are expressed in terms of International Units (IU)
of xylanase activity, as applied per ton of pulp. The commercial enzymes are prepared to deliver a
specific dose of lU's per kilogram to enable application on about a 1 kg per ton of pulp basis.
4.7.1 Number of Installations
The exact number of mills running enzyme trials or using enzymes for production quantities is
not known but it is believed that interest is high at this time. Given that the modifications necessary to
accommodate enzyme treatment are quite minor, it would not be surprising to find large numbers of mills
investing in small quantities of enzymes to carry out their own trials. Indications of the extent of interest
in enzymes can be seen in the following observations:
• At the Nonchlorine Bleaching Conference in March 1992, mills in Canada and Finland were
the only ones reporting use of enzymes in full-scale;
Page 4-78
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
Figure 4-17. Impact of xylanase treatment on AOX.
Figure 4-18. Impact of xylanase treatment on brightness.
Source: Senior and Hamilton, 1992.
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Pollution Prevention in Pulp & Paper Section Four - Pulping
• Juracek and Paice (1993) estimated in August, 1992 that 10 mills were using enzymes in lull
scale commercial application and that 85 mills were running trials. Six of the 10 mills running
full time were in Europe and the other four were in Canada;
• In a paper published in September, Senior and Hamilton (1992) indicated that a "significant
number" of mills in North America and Scandinavia are running enzyme trials;
• At the TAPPI Pulping Conference in November, 1992, industry observers suggested that there
are more mills running full-scale trials than were prepared to disclose their process publicly.
Given that the biotechnology industry has only recently begun focussing on the pulp and paper
industry, it is likely that enzyme applications will be improved in the future, further enhancing their
attractiveness for mill usage. Thus, additional experimentation with enzymes and possible new uses can
be expected.
4.7.2 Costs and Economics
The equipment necessary to apply enzymes to pulp is quite modest. Estimates of the costs for
an enzyme delivery system and pH adjustment range from $10,000 to $100,000 (Juracek and Paice, 1993).
The enzyme cost per ton of pulp is variable and depends on the type, activity level, and recommended
application rate for the enzymes. Costs in the range of $5 to 10 per ton of pulp are probably appropriate,
which will be partially or completely offset by savings in chemical costs.
So far, the potential for recycling enzymes has apparently not been investigated.20 Enzymes are
washed from the pulp and eventually are destroyed hi the mill's normal recovery cycle. If a method was
discovered to recycle enzymes, their cost of use might decrease, thereby enhancing their attractiveness.
Still another factor that would add to enzyme's appeal would be their ability to operate at closer
to normal brownstock pH and temperature conditions. The necessity to lower the pH and temperature of
the pulp before enzyme application imposes additional costs.
20 Recovery of enzymes would have to take place in front of the mill's regular "recovery" process,
which results in concentration and destruction of solids via incineration.
Page 4-80
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Section Four - Pulping Pollution Prevention in Pulp & Paper
4.7.3 Pollution Prevention Potential
Enzymes are used primarily as a substitute brightening agent that permit the mill to cut back on
the application of other, mostly chlorine-based bleaching chemicals. As a replacement for chlorine hi the
first bleaching stage, the effectiveness of enzymes in reducing formation of chlorinated organics will be
approximately proportional to the percentage decrease in chlorine application that is possible.
4.7.3 Compatibility With Other Aspects of Mill Operations
In enzyme trials held to date, no detrimental impacts on pulp quality, strength or other pulp
characteristics have been found.
4.8 IMPROVED BROWNSTOCK WASHING
In recent years, substantial progress has been made in improving the efficiency of brownstock pulp
washing systems. These advances have been aimed at reducing effluent flows, improving energy
efficiency, and achieving better removal of dissolved lignin solids and spent liquor from the pulp. Since
liquor solids carried over to the bleach plant will compete with lignin remaining in the pulp fibers for
reaction, improved pulp washing can lower the consumption of bleaching chemicals and hence the
formation of chlorinated organics. Efficient washing is also a prerequisite for successful operation of an
oxygen delignification system.
Rotary vacuum washers represented the standard within the pulp industry until the 1980s, with
approximately 5,000 units operating in North America (Nelson, 1992). These washing systems create a
vacuum inside the drum to hold the fiber in place and draw the wash water through the pulp mat (see
Figure 4-19). Normally, a series of three or four washers are configured in a wash line and operated in
countercurrent fashion. This means that the filtrate from each washing stage is used as washwater for the
previous (less clean) stage (see Figure 4-3).
Current state-of-the-art washing systems may replace the vacuum pressure units with atmospheric
or pressure diffusion washers, belt washers, or pulp presses (see Figure 4-19). Pressure washers apply
external atmospheric pressure to the fiber mat. By removing the vacuum system from the Inside of the
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
Single-stage vacuum washer
Three-stage diffusion washer
Pressure washer
ft
PULP
FLOW
PERFORAUU PLAFE
Detail of diffusion washer
HOOD
PULP
OUT
Horizontal belt washer
Figure 4-19. Alternative pulp washing equipment.
Source: Smook, 1982.
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Section Four - Pulping Pollution Prevention in Pulp & Paper
drum, the manufacturers can install a sophisticated filtrate collection and recirculation system. In diffusion
systems, the washing takes place without dilution to low consistency, thereby avoiding the large flows of
recycled filtrate required in the drum washing system. By avoiding free-falling liquid streams, diffusion
washers also eliminate any problems with foaming and air entrapment. Belt washers pass the pulp under
a series of showers (usually at least seven) with no mixing stages in between. These washing advances
have been associated with reduced effluent flows, energy savings, and less foaming.
Improved lignin removal and energy efficiency is also being obtained through the application of
sophisticated computer control systems and by improved attention to proper operation. Experiments using
"neural networks" to determine the underlying relationships between washing variables such as mat
density, mat consistency, and sodium loss, permitted additional fine tuning of washer operations that
resulted in chemical and energy savings (Beaverstock and Wolchina, 1992).
The efficiency of brownstock washing is often measured in terms of the "sodium carryover" and
is indirectly measured as the amount of NajSC^ (saltcake) lost. According to one source, sodium losses
that in the 1970s were commonly in the range of 50 kg per ton have been reduced to 7 kg per ton in new,
-~ to-date pressure washing systems (Ontario Ministry of the Environment, 1988). Since it is lignin
solids, however, and not sodium that reacts with bleaching chemicals, it can be argued that sodium
carryover reductions do not necessarily imply reduced bleaching demand. Stromberg (1990) showed that
at the same sodium loss the dissolved organic solids loss (expressed as COD) varied by up to four times
in different mills. For this reason, it is preferable to now measure washing efficiency using a COD
reactivity test. Sodium losses, however, remain the principle design tool and are widely reported. The
difference is minor in practice, since modifications that reduce sodium losses also reduce COD losses.
4.8.1 Number of Installations
The importance of efficient brownstock washing is now well-recognized throughout the North
American pulp and paper industry. The extent to which the industry has adopted the various upgrade
options is difficult to estimate, however, since improvements can be made through several means,
including better utilization of existing equipment, onsite refurbishment of old equipment, or replacement
of equipment through new orders placed with vendors. One industry observer contacted vendors and
verified orders for 38 brownstock washing systems between January 1988 and mid-1991 (Pulliam, 1991).
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Pollution Prevention in Pulp & Paper Section Four - Pulping
Others confirm through discussion with vendors and paper companies that washing practices have been
considerably upgraded.
4.8.2 Costs and Economics
Costs for improved washing could range from negligible in the case of relatively simple
optimization of existing equipment to close to $20 million to completely upgrade the wash system in a
1,000 rpd mill. A recent case study evaluated modern washer systems from three established vendors and
found that capital costs for all three ranged from $10.2 to $12.3 million for a hypothetical mill (Ricketts,
1992).21 It should be noted that these costs include costs of constructing an appropriately-sized building
to house the equipment. Depending on the circumstances, it may be possible to utilize existing space.
Major operational requirements for each system are shown in Table 4-20 and net incremental
requirements compared to the existing 25-year old system are converted to dollar values in Table 4-21.
Each of the newer systems will require less electric power and/or steam. The Chemi-Washer system was
shown to result in operating savings of $4.67 per ton, the Compaction Baffle Filtration system saved $2.32
per ton, and the Drum Displacer saved $2.13 per ton. Additional cost savings will result from reduced
bleaching chemicals requirements. These costs were not shown in the analysis because the hypothetical
mill produces unbleached linerboard.
Due to their smaller size, pressurized diffusion washers are relatively easy to retrofit in an existing
mill.
4.8.3 Pollution Prevention Potential
Improved pulp washing will reduce the amount of organic lignin solids carried through to the
bleaching stages, thereby reducing the potential for formation of chlorinated organic compounds such as
21 The systems evaluated were: Black Clawson's Chemi-Washer, IMPCO's Compaction Baffle Filter,
and the Kamyr/Ahlstrom Drum Displacement Washer. Diffusion washers, which may be the best option
for a retrofit, were not considered in their analysis.
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-20
Major Operating Cost Items for Existing Washing Line
Versus Three Modern Alternatives - Hypothetical Mill Retrofit
Operating Cost Item
Connected horsepower, hp
Net steam savings, mlb/hr
Defoamer usage, Ib/odt
No. of pieces rotating equipment
(motor driven)
Washer facewire replacement
Existing
System
870
0
3.0
9
1 per yr
Chemi-
Washer
820
39.3
20
15
2 per yr
CB
Filters
910
12.6
1.0
13
none
Drum
Displacer
270
3.0
0.5
10
none
Source: Ricketts (1991).
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-21
Annual Incremental Operating Costs Saved for
Three Modern Alternative Washing Systems - Hypothetical Mill Retrofit
($1,000)
Incremental Cost Item
Power at $0.0525/kWhr
Steam at $3.50/1,000 Ib
Defoamer at $0.45/lb
Maintenance labor and materials for
facewire change
Total annual savings
Savings per odt ($)
Chemi-
Washer
$12
$1,555
$118
($60)
$1,225
$4.67
CB
Filters
($12)
$370
$236
$15
$609
$2.32
Drum
Displacer
$158
$88
$297
$15
$558
$2.13
Source: Ricketts (1991).
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Section Four - Pulping Pollution Prevention in Pulp & Paper
dioxin and furan. Improvements in conventional pollutants such as BOD5, COD, resin acids, and color
can also be expected.
Researchers from Westvaco have published data that show the potential impacts of improved
washing in the bleach plant on formation of 2,3,7,8-TCDD. In one set of mill trials, the brownstock wash
volume was first increased above the normal rate and then decreased (Hise and Hintz, 1989). Results are
shown in Table 4-22. With poor washing, C-stage pulp had a dioxin reading of 7.3 ppt while D2-stage
pulp had a reading of 9.3. When washing was improved (by increasing volume) C-stage pulp dioxin
readings fell to 3.3 ppt and dioxin in D2-stage pulp was non-detectable. It should be noted here, however,
that the relative amounts of wash water used may not be a good predictor of the washing efficiency of
state-of-the-art versus conventional washing systems.
The authors of the above study also measured the impact of surfactant use and ethanol wash
during laboratory washing of pulp. Surfactants presumably could assist in the removal of reacted lignin
solids, while ethanol is a more potent washing fluid. The results showed that surfactants reduced TCDD
and TCDF levels in chlorination-stage pulp by 20 and 20 to 25 percent, respectively. Ethanol washing,
further, reduced both TCDD and TCDF levels by approximately 80 percent. It seems unlikely that ethanol
addition would be desirable, however, since it would add volatile components and BOD5 to the effluent
Improved washing also removes highly colored material and some of the persistent, non-
biodegradable black liquor fraction which would otherwise be discarded.
4.8.4 Impacts on Other Aspects of Mill Operations
Recovery Boiler Operations
As with other technologies that result in further removal of lignin solids from the pulp, upgraded
brownstock washing increases the quantity of solids recovered from the condensed pulping effluent. These
additional solids are processed in the recovery boiler, which recovers their heat value, instead of
continuing to the bleach plant and eventually being discharged to the wastewater treatment system. The
processing of these additional solids in the boiler may marginally impact air emissions and solid waste
byproducts from the rest of the recovery process (grits and dregs).
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Pollution Prevention in Pulp & Paper
Section Four - Pulping
TABLE 4-22
Impacts of Improved Washing Practices on
Formation of Dioxin
Bleaching stage
C-Stage pulp
E-Stage pulp
DrStage pulp
D2-Stage pulp
2,3,7,8-TCDD (ppt)
"Good" Washing
3.3
2.1
2.5
ND (4.2)
"Poor" Washing
7.3
6.3
8.1
9.3
ND non-detectable
Source: ffise and Hintz (1989).
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Section Four - Pulping Pollution Prevention in Pulp & Paper
Energy Requirements
According to available information, newer pressure washing systems are more energy efficient than
previous types. The recovery of additional solids through the boiler will result in increases in boiler
energy output.
Implementation of Further Pollution Prevention Technologies
Efficient brownstock washing is necessary to accomplish effective removal of solids prior to
oxygen delignification, and is considered a prerequisite to introduction of an oxygen stage. Improved
washing is also an essential part of the zero effluent mill proposed by Albert (1992).
4.9 CLOSED SCREEN ROOM
The term "closed screen room" refers to modification of the brownstock screening system to use
only recycled unbleached white water from the decker as dilution water for the screeners and wash water
for the washers. Complete closure of the screen room with little excess flow to sewer is difficult to
achieve but will yield significant environmental benefits, including reduction of flow and loadings of
BOD5 and other pollutants.
Costs for closed screen room configuration in a new installation will range from $10 to $15
million. Costs in a retrofit situation will be site-specific.
4.10 MISCELLANEOUS PULPING TECHNOLOGIES
A number of additional technologies capable of reducing pollution generation in the pulping area
are available and have been discussed to varying lengths in the literature. Those presented above currently
represent the most promising and/or the most widely adopted options. The additional methods have in
some cases been discussed for several years without making it into full-scale mill operation, or have seen
only limited full-scale operations. Problems have been encountered in either scaling up the process from
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Pollution Prevention in Pulp & Paper Section Four - Pulping
lab-scale to pilot plant and full-scale operation, and for some facilities the economics have just not been
favorable.
4.10.1 The Lignox Process
Lignox is a proprietary process developed by Eka Nobel in Sweden. The process involves a two-
stage treatment of pulp, normally sandwiched between an oxygen delignification stage and a C1O2 or non-
chlorine bleaching sequence. The first stage, referred to as "X", involves the application of the chelating
agent EOT A to the pulp at a rate of about 2 kg per ton. This allows a specific profile of the trace metals
contained in the pulp to be used in the application of the second "P" stage, which is an alkaline peroxide
treatment. The process has shown the following results:
• When applied to a pulp with kappa 18, application of EDTA and peroxide at 25 kg per ton can
produce 70 ISO brightness;
• When combined with extended cooking, a brightness of 75 ISO was attainable with peroxide
charge of 30 kg per ton.
• Addition of ozone and an additional peroxide stage has been shown to produce a brightness of
more than 85 ISO for softwoods and 89 ISO for hardwoods (Basta et al, 1992.)
The Swedish mill at Aspa utilizes the Lignox process to produce totally chlorine free (TCF) kraft
pulp. The mill produces a paper with brightness of 70 to 75 ISO with good strength characteristics. The
plant currently operates at full capacity at around 110,000 ADt/yr (Malinen, 1992.) Although other mills
appear capable of incorporating the LIGNOX process in order to produce chlorine-free paper, the costs
associated with the high peroxide application rates would tend to discourage it.
4.10.2 Solvent pulping
Three solvent-based kraft alternative pulping processes have been developed that avoid the use
of sulfur-based chemicals. Because of this, they are characterized by low recovery costs. Two of these
have been developed in Germany and one in North America.
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Section Four - Pulping Pollution Prevention in Pulp & Paper
Alcell® Pulping
The Alcell® process has been developed by Repap Enterprises (Valley Forge, PA). Alcell stands
for alcohol-cellulose, and it uses an acidic water-alcohol mixture at 195 °C to solubilize and remove lignin
from hardwood pulps. The process supposedly produces kraft-equivalent pulp with similar strength and
optical properties (Harrison, 1991).
The simplified Alcell® recovery process reportedly makes the process viable at one-quarter the
scale of a modern kraft mill. A methanol distillation tower replaces the recovery furnace, lime kiln,
recausticizer, and other kraft recovery equipment. The three main steps in recovery are: 1) lignin
precipitation (proprietary Alcell® technology), 2) methanol recovery, and 3) byproduct recovery.
The byproducts are an essential part of the economics of the process. They include Alcell® lignin,
furfural, and a mix of hemicellulose sugars (Jamieson, 1991). Furfural is a commodity chemical that can
be easily sold into existing markets for use in engineering plastics and other applications. The
carbohydrate solution can be processed into a suitable supplement for animal feed. The mill must find
markets for the lignin, however, that earn a credit above its value as hog fuel. Because the process lacks
a recovery boiler, it is a heavy net energy consumer and the mill cannot afford to burn the lignin for
energy. The lignin has properties different from other kraft lignins, namely the absence of sulfur or
sodium, that make it interesting. While numerous uses for the lignin are being investigated, so far the
uses are mostly experimental. The applications tried so far include: a replacement for phenol
formaldehyde resin in building board manufacturing, a tackifying agent in rubber compounding, an
additive for agrochemicals, and as a water reducer and plasticizer in concrete.
An Alcell® demonstration plant was started up in Newcastle, New Brunswick in 1989 to produce
15 tons per day at a cost of $95 million. The success of this project prompted Repap to begin work on
a full-scale, 300 tpd facility which is scheduled for startup in 1993 at the same Newcastle site. So far,
this is the only pilot or full-scale installation to be built. The developers foresee opportunities in regions
with small hardwood resources and for reopening small kraft or sulfite mills that have closed due to
environmental or high cost reasons (Pye, 1990).
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Pollution Prevention in Pulp & Paper Section Four - Pulping
Organocell Pulping
Organocell has emerged in Germany as an alternative to kraft, which has never established itself
as an industry in that country because of concerns over odor and safety. The process uses a soda-AQ-
methanol liquor and currently is used only to produce fluff pulp (used as an absorbent in diapers, hygienic
products, etc). The first full-scale application is at a converted sulfite mill in Kelheim, north of Munich.
The mill currently produces 60,000 tpy of pulp from spruce chips generated by some 500 nearby sawmills,
and will soon upgrade to 150,000 tpy capacity.
The organocell mill resembles a kraft mill but requires additional features to protect it from the
dangers of methanol cooking. These include thicker digester walls to withstand the high pressure of the
methanol vapor (Fleming, 1993). It includes a full recovery furnace plus a methanol recovery unit.
Explosion-proof electrical equipment will also be necessary.
Fleming (1993) suggests that capital costs for a greenfield organocell mill will be similar to kraft,
but that operating costs will be higher, in part due to the use of AQ at a cost of some $17 per ton.
Alkaline Sulfite Anthraquinone Methanol (ASAM)
ASAM pulping is a low-alkalinity process developed at Heidelburg University. The process is
reported to be capable of producing higher-strength pulps than kraft with good bleachability and at
somewhat higher yields. Although classified as a solvent pulping method, it is not typical in that
methanol is used as an additive only. The delignification is performed using a mixture of sodium
hydroxide, sodium carbonate, sodium sulfate, AQ, and methanol.
ASAM has a liquor recovery cycle similar to kraft pulping with three separate loops to recover
methanol, caustic, and sodium sulfite. The capital costs are expected to be about 10 to 20 percent higher
than for kraft. Operating costs will also be higher; the AQ requirements alone amount to about $17 per
ton (Fleming, 1993).
So far, the only pilot plant in operation is a 5 tpd mill in southern Germany.
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Section Four - Pulping Pollution Prevention in Pulp & Paper
4.10.3 Polysulfide Cooking
The beneficial impacts of polysulfides on the kraft cooking process have been known for some
time. These include the ability to increase yield at the same kappa number or to obtain the same yield
at a lower kappa number. Polysulfides accomplish this by stabilizing the end groups of cellulose and
hemicellulose polymer chains against "peeling" reactions (Smook, 1989.) In terms of environmental
benefits, the key impact is a reduction in the thermal value of black liquor solids. This reduction means
that an increased boiler load, such as that resulting from oxygen delignification or extended cooking, can
be accommodated.
Until recently, it was necessary to inject cooking liquor with sulfite to obtain polysulfides. This
technique increases the sulfidity of the liquor, however, and causes an increase in air emissions. Patented
processes have now been developed that can partially oxidize the kraft white liquor. The oxidation
converts sulfides in the liquor to polysulfides without the detrimental effect on air pollutants. The thermal
load decrease is reported to be in the neighborhood of 6 percent, which should be sufficient to offset the
extra solids load obtained with oxygen delignification.
Polysulfide liquor (known as orange liquor) is produced in an oxidation unit installed between the
white liquor storage and the kraft digester. A patented catalyst used in the oxidization reactor is
responsible for the white liquor oxidation (Lightfoot, 1990).
Polysulfide cooking has also been used in combination with anthraquinone (AQ) catalysis, where
synergistic effects have been noted (Lightfoot, 1990). As shown in Section 4.5, AQ addition by itself can
also improve the yield of the kraft cooking process. In combination with polysulfide cooking, however,
the yield improvements are more than additive. Lightfoot cites data from the Shirakawa mill in Japan
which indicate that polysulfide and AQ addition individually had increased yield by 1.1 percent. In
combination, however, an increase of 3.1 percent was recorded.
Although the technology that has been available for several decades, polysulfide cooking has been
viewed by the industry as unusable because the process has been associated with losses in strength.
Researchers recently showed that when polysulfide cooking is combined with extended modified cooking,
pulp yield and strength properties can be improved simultaneously. When compared with bleachable
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Pollution Prevention in Pulp & Paper Section Four - Pulping
conventional kraft pulps, the pulp combining extended delignification and polysulfide cooking produced
a comparable property in terms of strength, kappa number and yield (Jiang, 1992.)
As of 1990, the polysulfide process developed by Mitsubishi and the Chiyoda Corporation was
installed at 6 mills. Four of these were in Japan, and one each was located in Norway and Austria (EPA,
1990a). Installation data for the process developed by Mead Corporation (MOXY process) was not
available. Estimates of capital costs for installation of the Chiyoda polysulfide system are shown in Table
4-23.
4.10.4 Demethylation
Demethylation of pulp has been shown in the laboratory to increase the number of reaction sites
where delignification can take place (Pryke, 1985). If a commercial system could be implemented, a
significant increase in the amount of delignification occurring prior to chlorination may be feasible.
Demethylation followed by a treatment with 2 percent H2O2 on pulp produced a pulp with kappa number
7.5.
The demethylation process has been conducted by treating pulp with a solution of potassium
thiophenoxide in diethylene glycol at 200° C. Due to the complexity and extreme conditions under which
the demethylation reaction has been induced, however, no commercial process has been developed so far.
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Section Four - Pulping
Pollution Prevention in Pulp & Paper
TABLE 4-23
Capital Costs for Chiyoda Polysulfide Process
Mill Size
(ADT/day)
500
750
1,000
Cost
($millions)
1.28
1.89
2.23
Source: Chiyoda Corporation; cited in EPA (1990a).
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Pollution Prevention in Pulp & Paper Section Four - Pulping
SECTION FOUR REFERENCES
Albert, 1992. Richard J. Albert. "The Effluent-Free Kraft Pulp Mill Technical and Economic
Considerations," Proceedings, 7992 TAPPI Pulping Conference, Boston, MA, November 1992.
Anderson, 1992. Ross Anderson. "Peroxide Delignification and Bleaching," Proceedings, Non-Chlorine
Bleaching Conference, Hilton Head SC, March 1992. Available from Miller-Freeman
Publications, San Francisco.
API, 1992. American Paper Institute. 7992 Statistics of Paper, Paperboard, & Wood Pulp. New York.
Bansal, 1987. Ravinder K. Bansal. "On-Site Pressure Swing Adsorption Systems for the Pulp and Paper
Industry," Proceedings, 7987 TAPPI Oxygen Delignification Conference, p. 151.
Basta et al., 1992. J. Basta, L. Anderson, W. Hermansson. "LIGNOX and Complementary
Combinations," Proceedings, Non-Chlorine Bleaching Conference, Hilton Head SC, March 1992.
Available from Miller-Freeman Publications, San Francisco.
Bauerlin et al., 1991. C.R. Bauerlin, M.H. Kirby, G. Berndt. "Vacuum Pressure Swing Adsorption
Oxygen for Oxygen Delignification," TAPPI Journal, May 1991, p. 85.
Beaverstock and Wolchina, 1992. Malcolm Beaverstock and Kenneth Wolchina. "Neural Network Helps
G-P Ashdown Mill Improve Brownstock Washer Operation," Pulp and Paper, September 1992,
p. 134.
Beloit Corp., 1990. Cited in U.S. EPA, Summary of Technologies for the Control and Reduction of
Chlorinated Organics From the Bleached Chemical Pulping Subcategories of the Pulp and Paper
Industry. U.S. EPA, Office of Water Regulations and Standards. Office of Water Enforcement
and Permits. April 27, 1990.
Berry et al., 1991. R.M. Berry, C.E. Luthe, R.H. Voss, P.E. Wrist, P. Axegard, G.Gellerstedt, P.O.
Lindblad, and I. Popke. "The Effects of Recent Changes in Bleached Softwood Kraft Mill
Technology on Organochlorine Emissions an International Perspective," Proceedings, CPPA
Spring Conference, Whistler, B.C., May 1991.
Blain, 1992. T.J. Blain. "Anthraquinone Pulping: Fifteen Years Later," Proceedings, 7992 TAPPI
Pulping Conference, November 1992, Boston, MA.
Bostrb'm and Hillstrb'm, 1992. "Status report from the Chemrec recovery booster at Frovifors,"
Proceedings, 7992 International Chemical Recovery Conference, CPPA, Seattle, WA.
Brenner and Pulliam, 1992. Lee Brenner and Terry Pulliam. "A Comprehensive Impact Analysis of
Future Environmentally Driven Pulping and Bleaching Technologies," Proceedings, 7992 TAPPI
Pulping Conference, Boston, MA, November 1992.
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Section Four - Pulping Pollution Prevention in Pulp & Paper
SECTION FOUR REFERENCES (cont)
Byrd and Knoernschild, 1992. Medwick V. Byrd and Kevin J. Knoernschild, "Design Considerations for
an Ozone Bleaching System," TAPPI Journal, May 1992, p. 101.
Clement, 1993. John L. Clement. "Recovery Boiler Capability to Accommodate Alternative Kraft Mill
Processes," Proceedings, International Symposium on Pollution Prevention in the Manufacture
of Pulp and Paper, August 18-20, 1992, Washington, DC. U.S. Environmental Protection
Agency, Office of Pollution Prevention and Toxics. EPA-744R-93-002. February 1993.
Deal et al., 1991. Richard Deal, Richard Hendrickson, Wiliam Lewis. "Onsite Oxygen Plants Can
Reduce Pulp Mill Costs for Volume Users," Pulp and Paper, September 1991, p. 141.
Deal, 1991. Howard Deal. "Environmental Pressure Causes Changes in Bleaching Technologies,
Chemicals," Pulp and Paper, November 1991, p. 110.
Elliot, 1989. R.G. Elliot. "Experience with Modified Continuous Cooking," presented at University of
Washington, Pulp and Paper Foundation Annual Meeting, May 23-24, 1989.
Farrell, 1992. Roberta L. Farrell. "Status of Enzyme Bleaching R&D and Mill Work," Proceedings
Nonchlorine Bleaching Conference, Hilton Head SC, March 1992. Available from Miller-Freeman
Publications, San Francisco.
Ferguson, 1992b. Kelly Ferguson. "Union Camp Begins Ozone Era with New Kraft Bleaching Line at
Franklin, VA," Pulp and Paper, November 1992, p. 42.
Ferguson, 1992a. Kelly Ferguson. "P&W Starts Up Alabama Pine Pulp Mill with Plans to Produce
430,000+ tpy," Pulp and Paper, March, 1992. p. 71.
Fleming, 1993. Bruce I. Fleming. "Alternative and Emerging Non-Kraft Pulping Technologies,"
Proceedings, International Symposium on Pollution Prevention in the Manufacture of Pulp and
Paper, August 18-20, 1992, Washington, DC. U.S. Environmental Protection Agency, Office of
Pollution Prevention and Toxics. EPA-744R-93-002. February 1993.
Galloway et al, 1989. L.R. Galloway, P.I. Helminen, D.N. Carter. "Industry's Effluent Problems
Spawn New Engineering Technology and Design," Pulp & Paper, September 1989, p. 91.
Gotaverken, 1993. "Recovery Boiler Bottleneck Eliminated at Frovifors Mill," TAPPI Journal, Supplier
Update, March, 1993, p. 249.
Gullichsen, 1991. Johan E. Gullichsen. "Means to Reduce Effluent Pollution of Kraft Mills,"
Proceedings 7997 TAPPI Environmental Confernece, San Antonio, Texas, Book 1 pp 185-190.
TAPPI Press, Atlanta, GA.
Harrison, 1991. Andy Harrison. "Repap Produces High-Quality Pulp at Newcastle with Alcell Process,"
Pulp and Paper, February 1991, p. 116.
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Pollution Prevention in Pulp & Paper Section Four - Pulping
SECTION FOUR REFERENCES (cont)
Harder, 1978. Nils Hartler, "Extended Delignification in Kraft Cooking A New Concept," Svensk
' Paperstidning. 81(15), (October 25, 1978), p 483.
Heimburger et al., 1988a. Stanley A. Heimburger, Daniel S. Blevins, Joseph H. Bostwick, and G. Paul
Donnini, "Kraft Mill Bleach Plant Effluents: Recent Developments Aimed at Decreasing Their
Environmental Impact, Part 1," TAPPI Journal, October 1988, p. 51.
Hise and Hintz, 1989. Ronnie G. Hise and Harold L. Hintz. "Effect of Brownstock Washing on
Formation of Chlorinated Dioxins and Furans During Bleaching," Proceedings, 1989 TAPPI
Pulping Conference.
Idner, 1988. Kristina Idner. "Oxygen Bleaching of Kraft Pulp: High Consistency vs. Medium
Consistency," TAPPI Journal, February, 1988, p.47.
Jamieson, 1991. Scott Jamieson. "Alcell Pulping: World Class Research Right Here in Canada," Pulp
and Paper Canada, 92:3 (1991), p. 16.
Jiang, 1992. Jian Er Jiang. "Extended Modified Cooking With Polysulfide For Simultaneous Pulp Yield
and Strength Improvement," Proceedings, 7992 TAPPI Pulping Conference, Boston MA,
November 1992, p.683
Johnson, 1992. Anthony P. Johnson. "Worldwide Survey of Oxygen Bleach Plants," Proceedings,
Nonchlorine Bleaching Conference, Hilton Head, SC March 1992. Available from Miller-Freeman
Publications, San Francisco.
Johnson, 1993. Anthony P. Johnson. "Oxygen Delignification Systems Flourish as Mills Push for Lower
Kappa Number," Pulp & Paper, March, 1993, p. 103.
Jurasek and Paice, 1993. Lubomir Juracek and Michael Paice. "Saving Bleaching Chemicals and
Minimizing Pollution with Xylanase," Proceedings, International Symposium on Pollution
Prevention in the Manufacture of Pulp and Paper, August 18-20, 1992, Washington, DC. U.S.
Environmental Protection Agency, Office of Pollution Prevention and Toxics. EPA-744R-93-002.
February 1993.
Kamyr, Inc., 1990. Cited in U.S. EPA, Summary of Technologies for the Control and Reduction of
Chlorinated Organics From the Bleached Chemical Pulping Subcategories of the Pulp and Paper
Industry. U.S. EPA, Office of Water Regulations and Standards. Office of Water Enforcement
and Permits. April 27, 1990.
Kumar et al., 1992. Kumar, K., H.-m. Chang, H. Jameel, N.H. Shin. "Elemental Chlorine-Free (ECF)
and Total Chlorine-Free (TCP) Bleaching of RDH Hardwoods," Proceedings, 7992 TAPPI Pulping
Conference, November 1-5, Boston, MA. p. 169.
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Section Four - Pulping Pollution Prevention in Pulp & Paper
SECTION FOUR REFERENCES (cont)
Liebergott et al., 1992a. Norman Liebergott, Barbara van Lierop, Anastasios Skothos. "A Survey of the
Use of Ozone in Bleaching Pulps, Part 2," TAPPI Journal, February 1992, p. 117.
Liebergott et al., 1992b. Norman Liebergott, Barbara van Lierop, Anastasios Skothos. "The Use of
Ozone in Bleaching Pulps," Proceedings, 7992 TAPPI Environmental Conference, Richmond VA,
April 1992.
Lightfoot, 1990. W.E. Lightfoot. "New Catalyst Improves Polysulfide Liquor Makeup, O2
Delignification," Pulp and Paper, January 1990, p. 88.
Macleod, 1992. Martin Macleod. "Extended Cooking in the Mills," Proceedings, Nonchlorine Bleaching
Conference, Hilton Head, SC, March 1992. Available from Miller-Freman Publications, San
Francisco.
Malinen, 1992. Raimo Malinen. "Chlorine-Free Bleaching: State of the Art in Scandinavia," Proceedings,
Non-Chlorine Bleaching Conference, Hilton Head SC, March 1992. Available from Miller-
Freeman Publications, San Francisco.
McCubbin et al., 1991. Neil McCubbin, Howard Edde, Ed Barnes, Jens Folke, Eva Bergman, Dennis
Owen. "Best Available Technology for the Ontario Pulp and Paper Industry," Rep. ISBN 7729-
9261-4, Ontario Ministry of the Environment, Toronto, Ontario.
McCubbin, 1992. Memorandum from Neil McCubbin to ERG concerning extended cooking. November
18, 1992.
McCubbin, 1993. Neil McCubbin. "Costs and Benefits of Various Pollution Prevention Technologies
in the Kraft Pulp Industry," Proceedings, International Symposium on Pollution Prevention in the
Manufacture of Pulp and Paper, August 18-20, 1992, Washington, DC. U.S. Environmental
Protection Agency, Office of Pollution Prevention and Toxics. EPA-744R-93-002. February
1993.
McDonough, 1986. Thomas J. McDonough. "Oxygen Bleaching Practices," TAPPI Journal, June 1986,
p. 46.
Miller, 1992. Bill Miller. "Tutorial: Process Technology, Machinery, Advantages & Disadvantages,"
Proceedings, Nonchlorine Bleaching Conference, Hilton Head SC, March 1992. Available from
Miller-Freeman Publications, San Francisco.
Nelson, 1992. Phil Nelson. "Mills Improve Pulp Washer Performance with New Rotary Vacuum Filter
Design," Pulp and Paper, November 1992, p. 101.
Norden et al., 1992. Norden, S., M. Dahl and R. Reeves. "Bleaching of Extremely Low Kappa Southern
Pine, Cooked by the SuperBatch Process," Proceedings, 7992 TAPPI Pulping Conference,
November 1-5, Boston, MA. p. 159.
Page 4-99
-------
Pollution Prevention in Pulp & Paper Section Four - Pulping
SECTION FOUR REFERENCES (cont)
Nutt et al., 1992. . "Development of an Ozone Bleaching Process," Proceedings, 7992 TAPPI Pulping
Conference, Boston, MA November 1992.
Nutt et al., 1993. W. E. Nutt, B.F. Griggs, S.W. Eachus, M.A. Pikulin. "Developing an Ozone Bleaching
Process," TAPPI Journal, March, 1993, p. 115.
Ontario Ministry of the Environment, 1988. Kraft Mill Effluents in Ontario. Municipal-Industrial Strategy
for Abatement. March 1988.
Paper Age (1990). no author. March, 1990, p. 23.
Phillips et al., 1993. Richard B. Phillips, Lindsay M. Lancaster, Jean J. Renard, Caifang Yin. "The
Effects of Alternative Pulping and Bleaching Processes on Product Performance - Economic and
Environmental Considerations," Proceedings, International Symposium on Pollution Prevention
in the Manufacture of Pulp and Paper, August 18-20, 1992, Washington, DC. U.S.
Environmental Protection Agency, Office of Pollution Prevention and Toxics. EPA-744R-93-002.
February 1993.
Pye, 1990. E. Kendall Pye. "The Alcell Process A Proven Alternative to Kraft Pulping," Proceedings,
7990 TAPPI Pulping Conference, p. 991.
Reeve, 1990. Douglas W. Reeve. "Chlorine Dioxide Delignification Process Variables," Proceedings,
7990 TAPPI Pulping Conference, p. 837.
Ricketts, 1992. Drew Ricketts. "Three BSW Systems Studied to Find Best Fit for Mill Upgrade Project,"
Pulp and Paper, September 1991, p. 94.
Senior and Hamilton, 1992. David Senior and Janice Hamilton. "Biobleaching with Xylanases Brings
Biotechnology to Reality," Pulp and Paper, September 1992, p. 111.
Shackford, 1992. Lewis Shackford. "Commercial Implementation of Ozone Bleaching Technology,"
Proceedings, Nonchlorine Bleaching Conference, Hilton Head, SC March 1992. Available from
Miller-Freeman Publications, San Francisco.
Shin et al. 1990. N.H. Shin, M. Sundaram, H. Jameel, H. Chang. "Bleaching of Softwood RDH Pulps
With Low/No Chlorine Bleaching Sequences," Proceedings, 7990 TAPPI Environmental
Conference, p. 549.
Singh, 1982. Rudra P. Singh. "Ozone Replaces Chlorine in the First Bleaching Stage Advances in
Ozone Bleaching, Part 1," TAPPI Journal, February 1982.
Smook, 1982. Gary A. Smook. Handbook for Pulp and Paper Technologists. (TAPPI/CPPA,
Atlanta/Montreal).
Page 4-100
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Section Four - Pulping Pollution Prevention in Pulp & Paper
SECTION FOUR REFERENCES (cont)
SRI International, 1988. Stanford Research Institute. Chemical Economics Handbook Marketing
Research Report: Air Separation Gases.
Stromberg, 1990. Bertil Stromberg, 'Washing for Low Bleach Chemical Consumption," Proceedings,
7990 TAPPI Pulping Conference, p. 883.
Stromberg, 1993. Bertil Stromberg. "Low Kappa Continuous Cooking," Proceedings, International
Symposium on Pollution Prevention in the Manufacture of Pulp and Paper, August 18-20, 1992,
Washington, DC. U.S. Environmental Protection Agency, Office of Pollution Prevention and
Toxics. EPA-744R-93-002. February 1993.
Tench and Harper, 1987. Larry Tench and Stuart Harper. "Oxygen Bleaching Practices and Benefits
An Overview," Proceedings, 7987 TAPPI International Oxygen Delignification Conference, p.
1.
van Lierop and Brown, 1987. Barbara van Lierop and Gordon Brown. "Varying the Purity of Oxygen
Gas Used in Oxygen Delignification and Oxidative Extraction Stages," Proceedings, 79S7 TAPPI
International Oxygen Delignification Conference, p. 133. TAPPI Press, Atlanta.
:ey, et al., 1990. D.L. Whitley, J.R. Zierdt, D.J. Lebel. "Mill Experiences with Conversion of a
Kamyr Digester to Modified Continuous Cooking," TAPPI Journal, January 1990, pp. 103-108.
Page 4-101
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Section Five Bleaching Pollution Prevention in Pulp & Paper
SECTION FIVE
POLLUTION PREVENTION TECHNOLOGIES IN BLEACHING OPERATIONS
This section examines pollution prevention technologies that can be implemented in the bleaching
areas of the mill. The "bleaching areas" are defined according to the traditional distinction as
those stages that include and follow the first chlorination stage in a conventional kraft mill As
discussed previously, however, with the advent of numerous pre-chlorination stages and the
replacement of chlorine-based chemicals, these distinctions are becoming blurred.
Most of the technologies discussed in this section are aimed at reducing dioxin, AOX, and
chloroform. With the exception of chlorine dioxide substitution, they all result in greater
recycling of wood organics and associated chemicals through the recovery boiler. Their impact
on reducing most chlorinated and unchlorinated pollutants is roughly proportional to the
reductions in elemental chlorine usage that they facilitate.
This section begins with a discussion of conventional kraft mill bleaching practices in Section 5.1.
Section 5.2 discusses high chlorine dioxide substitution as a means of reducing pollution, and
Section 5.3 examines split addition of chlorine. Sections 5.4 and 5.5 provide information on
oxygen and peroxide reinforcement of the alkaline extraction stages, respectively.
5.1 CONVENTIONAL KRAFT PULP BLEACHING
Bleaching is a process that lightens or whitens the cellulose fibers in the pulp through chemical
actions. The bleaching action can involve either oxidative or reductive processes (or both) that increase
the solubility of color bodies in the pulp, making them easier to remove. The bleaching of pulp is usually
performed with two interrelated objectives: (1) whitening the fibers, which makes them more suitable for
printing and other applications; and (2) increasing the permanence of the whiteness (to avoid yellowing
or aging of final products) .u The ease with which pulp can be bleached depends on the amount of lignin
that remains in the fibers following pulping. As indicated above, chemical pulping removes up to 90
1 Bleaching chemicals and conditions may also be controlled in order to influence pulp properties
such as absorbency and porosity.
2 Bleaching is also used to remove dirt from the pulp, to the extent that dirt is in the form of uncooked
fiber bundles (or "shives") that are harder to bleach. (Bleaching does not effectively remove carbon-based
dirt particles.) In conventional pulping and bleaching, it has often been necessary to add excess chemicals
in order to eliminate shives. One advantage of the newer technologies such as extended cooking and
oxygen delignification is that shives are greatly reduced.
Page 5-1
-------
Pollution Prevention in Pulp & Paper _ Section Five Bleaching
percent of the lignin present in the wood chips. Chemical bleaching applies progressively more selective
agents in an effort to remove more and more of the remaining lignin fractions without further damaging
the cellulose.
If the cooking reaction is extended in an effort to remove additional lignin, damage will result to
the cellulose portions of the fiber, causing sharp decreases in pulp yield and paper strength characteristics.
For this reason, the chemical cook is closely controlled so as to achieve the lowest possible kappa number
(lignin content) while holding yield losses to an acceptable level.
The pulp kappa number is used as the primary control parameter up until the point where the pulp
leaves the brownstock washers. After the pulp enters the bleaching stage, however, attention turns to
measures of pulp brightness. Brightness is a measure of the amount of light reflected from the pulp. A
number of specific brightness measurements are available, all of which measure the proportion of incident
light that is reflected from the pulp. For example, a reading of 0 indicates that all incoming light is
completely absorbed by the pulp, while a reading of 100 indicates perfect reflectance.3 Unbleached kraft
pulp (brownstock) registers brightness readings in the range of 15 to 30 percent. The sulfite process
produces pulps with a brightness in the range of 50 to 65 percent. For printing purposes, most pulps are
bleached to a brightness level ranging from about 70 percent, which is somewhat creamy in appearance,
to "full brightness" or maximum levels of between 88 and 94 percent.
Traditionally, mills that produce market pulp (i.e., pulp sold to other mills for papermaking) have
had to meet high brightness standards based on the requirements of pulp buyers. It is not clear, however,
to what extent market pulp bleached to lower brightness levels would be accepted. Brightness standards
for market pulp are higher, for instance than most integrated mills require for their own paper products.
Many integrated mills reportedly use pulp bleached to a level of 80 to 88 percent (depending on the
source).
3 At least three different measurement techniques are in use around the world. Along with the regions
where they are most widely used, these are: GE brightness (U.S.); Elephro brightness (Canada); and ISO
brightness (Scandinavia). Throughout most of this report, brightness is reported using ISO measures,
(most often used in the technical literature) unless otherwise indicated. All three are measured in a similar
fashion, and there is usually little more than a single point difference between them on a scale of 0-100.
As a general rule, the following applies: 92 GE = 91 Elephro = 90 ISO.
Paee 5-2
-------
Section Five - Bleaching Pollution Prevention in Pulp & Paper
Pulp bleaching is performed using a variety of chemicals and process conditions in a series of
stages known collectively as the bleaching sequence. Table 5-1 lists the common bleaching stages and
the conditions under which they are carried out. The shorthand abbreviations are used universally in the
industry to describe the bleaching sequence. For example, C represents a bleaching stage using elemental
chlorine, E is an extraction stage using caustic, and so on.
Successive stages are needed in the bleaching sequence to break down and remove lignin that
remains in the pulp following cooking, screening, and washing. Bonds between lignin and fiber are first
broken in the bleaching stages through the use of chemicals such as elemental chlorine or chlorine dioxide.
Then, in the extraction stage, the lignin is solubilized in caustic (sodium hydroxide) to facilitate its
removal, and washed from the pulp. It is the removal of lignin that causes the loss of color, or pulp
brightening. In the papermaking stages, additives such as titanium dioxide are used to make paper appear
still whiter.
Bleaching chemicals are distinguished by the nature of their chemical activity and by their
selectivity, i.e., their capacity to attack lignin while doing minimal damage to the cellulose fibers.
Typically, early stages in a bleaching sequence are designed to dissolve and then extract the bulk of the
lignin present in the brownstock. Because lignin concentrations in these stages are high, relatively non-
specific chemicals like chlorine can be used without causing significant damage to the pulp. During later
bleaching stages, when residual lignin concentrations are lower, more selective chemicals must be used.
The emphasis then shifts from removal of lignin to brightening of the small remaining amount of lignin.
Lignin removal and brightening stages are almost always separated by extraction stages, in which alkaline
chemicals are used to solubilize and facilitate removal of the lignin from the pulp. The E1 or first
extraction stage is almost as important as the first chlorination stage. After chlorination and washing (but
prior to extraction), approximately 30 to 50 percent of the chlorinated lignin is removed. Following
extraction, a full 80 to 90 percent of the lignin will have been removed.
Common bleaching sequences used to attain full brightness on softwood kraft pulp include
CEDED, CEHDED, or OCEDED. The first two of these begin with a chlorination stage. Chlorine is an
aggressive oxidant, and is traditionally used in the first stage to dissolve as much of the residual lignin
as possible. Chlorination is followed by alkaline extraction using caustic (NaOH). This neutralizes the
pulp and facilitates removal of the dissolved lignin solids during washing. The first sequence finishes with
a chlorine dioxide (C1O2) bleaching/brightening stage, followed by alkaline extraction, and a final
-------
"0
ffo
n
TABLE 5-1
Summary of Bleaching Chemicals
Chemical Sequence
Chlorine
ChJorine dioxide
Alkaline extraction with sodium
hydroxide
Hypochlorite
(sodium or calcium)
Peroxide
Hypochlorous acid
Nitrogen dioxide
Hydrosulfite
(sodium or zinc)
Ozone
Oxygen
Mixtures of chlorine and
chlorine dioxide (subscript
indicates lower quantity
chemical)
Sequential addition of chlorine
and chlorine dioxide
Alkaline extraction reinforced
with oxygen (O), sodium
hypochlorite (H), or hydrogen
peroxide (P)
Shorthand
C
D
E
H
P
L
N
Y
Z
O
DcOrQ,
D-C or C-D
EO. EH, Up
Symbol
C12
C102
NaOH
NaOCl or
Ca(OCl)2
H2O2 or NajO2
HC1O
NO2
NajS2O., or
ZnSjO,,
o,
02
Cl & C102
Cl & C107
NaOH & 02,
NaXX, or H2O2
Form
Gas dissolved in
water, 2-5%
chlorine on pulp
7 to 10 g/1 C102
solution in water
5 to 10% NaOH
solution
approx. 40 g/1
solution
2 to 5% solution
solution
solution or solid
gas
gas (for bleaching)
solution
NaOH solution,
reinforced with
approx. 10 Ibs
O/ton pulp
Process Conditions
pH: 2 or lower
Temp- 15-30 C
pH: 3.5-5
Temp: 60-80 C
pH: 10-11
Temp: 60-80 C
pH: 1 1 (must be buffered
due to release of HC1)
Temp: 20-60 C
pH 10.5
Temp: 70-80 C
pH 1.0 - 2.0
Temp: 90-120 C
Notes
Effective, economical delignification. Can cause loss of pulp
strength if used improperly.
High brightness without pulp degradation. Must be made at mil]
site.
Dissolves and removes reaction products from chlorination. Darkens
pulp-
Easy to make and use. Generated onsite from chlorine and caustic.
Can cause loss of pulp strength if used improperly. Linked to
formation of chloroform.
Easy to use. Low capital cost, but high chemical cost
Unstable
Rarely used.
Not commonly used.
Acid conditions
Alkaline conditions
Low cost but used in large quantities. Equipment expensive.
addition of oxygen to the extraction stage to achieve further
reductions in lignin
Source: Reeve (1987); OTA (1989).
-------
Section Five - Bleaching Pollution Prevention in Pulp & Paper
bleaching/brightening with chlorine dioxide. In the second sequence (CEHDED), an alkaline hypochlorite
stage is added following the first extraction stage; hypochlorite both brightens and solubilizes residual
lignin. In the third example (OCEDED), the pulp is "prebleached" with oxygen before undergoing the
traditional five stage chlorination-extraction process. As discussed above in Section 4.3, an oxygen
delignification stage can reduce the kappa number of the pulp entering the chlorination stage, thereby
reducing the amount of chlorine subsequently required.
The number of bleaching stages affects both the final brightness and the ability to consistently
control final brightness levels. A five stage CEDED bleaching sequence, for example, can bleach a given
pulp to a higher final brightness level, as well as ensure greater brightness consistency and stability, in
comparison with a CEH process.
Table 5-2 shows the most common bleaching sequences at U.S. pulp mills, based upon the results
of a 1990 EPA census. The most common sequences include CDE0D, DCDEOPD, DCEOPD, CDEDED,
and CEH, although there are as many as 90 additional unique sequences in use at smaller numbers of
facilities. Major trends in the industry in recent years have been: (1) elimination of the hypochlorite stage
(to reduce chloroform formation), and (2) the replacement of some chlorine in the first bleaching stage
with chlorine dioxide. Between the time of the 1985 EPA/Paper Industry's 104-Mill Study and the 1990
EPA Industry Census, 33 mills indicated that they would implement or increase C1O2 substitution, and an
additional 39 facilities reported that their intentions to do so over the period 1990 to 1993 (EPA, 1991).
This section continues with a description of chlorine dioxide substitution and a discussion of its pollution
prevention potential.
5.2 CHLORINE DIOXIDE SUBSTITUTION
The use of chlorine dioxide (CIO;,) as a bleaching agent became widespread in the 1960s, though
primarily in the latter stages of the bleach sequence (e.g., in the conventional CEDED sequence). Because
it is more selective towards lignin than chlorine, C1O2 is effective in removing the smaller amounts of
lignin that remain following chlorination without degrading the cellulose. Chlorine dioxide also saw use
in the 1960s in small fractions during the first bleaching stage to prevent pulp viscosity loss and associated
strength losses (McDonough, 1992). In recent years, however, attention has focused on increasing the
substitution rate of chlorine dioxide for chlorine in the first bleaching stage, due to its beneficial impacts
Page 5-5
-------
Pollution Prevention in Pulp & Paper
Section Five - Bleaching
TABLE 5-2
Most Common Bleaching Sequences
at U.S. Kraft Millsw
Bleach Sequence^1
C-E-H
C-E-HE-D
C-EO-HE-H-DE
CD-E-D-E-D
CD-E-H-D
CD-E-HE-D-E-D
CD-EO-D
CD-EO-H-D
CD-EOP-D
DC-EOP-D
DCD-EOP-D
Number of Mills with Bleach
Sequence
4
3
3
4
3
3
9
3
3
4
6
[al Bleaching sequences performed at three or more mills are listed. Approximately
90 other sequences are used at one or two mills for each sequence.
[b]
Key: C . . . Chlorination
E . . . . Extraction
D . . . . Chlorine dioxide
H Hypochlorite
O Oxygen
P . . . Peroxide
CD Chlorine dioxide substitution
EO Oxygen added to extraction stage
EOP . . . Peroxide and oxygen added to extraction stage
Source: EPA (1993).
Page 5-6
-------
Section Five - Bleaching Pollution Prevention in Pulp & Paper
on pulp and effluent quality. The replacement of up to 50-70 percent of the elemental chlorine in the first
bleaching stage with chlorine dioxide is now an accepted practice for environmental improvement and has
been widely adopted within the North American pulp and paper industry. Substitution of more than 50-70
percent of the chlorine with chlorine dioxide is commonly referred to as "high" substitution.
Chlorine dioxide substitution for elemental chlorine in the first bleaching stage can be as effective
as traditional bleaching in producing high strength, high brightness pulps. At substitution rates of 50 to
70 percent, chlorine dioxide can actually improve the efficiency of the delignification process over
conventional chlorine bleaching (McDonough, 1992). According to a recent survey of Canadian mills
using chlorine dioxide substitution, at substitution rates of 25 to 75 percent pulp brightness is not
significantly different than with chlorine bleaching. At substitution rates approaching 100 percent,
however, the efficiency or pulp yield declines slightly (McDonough, 1992) and brightness limits are
reported to be marginally lower (Pryke et al., 1992, cited in Reeve, 1993).
The pollution prevention potential of chlorine dioxide is related to the different ways it reacts with
lignin in the pulp, in comparison with chlorine. Reactions of chlorine-based compounds with lignin fall
into three categories: substitution, addition, and oxidation. The first two reactions result in the formation
of chlorinated organics, however, while oxidative reactions generally lead to fragmentation of the lignin
(Forbes, 1992). Compared to elemental chlorine, chlorine dioxide is more of an oxidative bleaching agent.
As a result, chlorine dioxide substitution increases the proportion of oxidative reactions, and leads to
reduced formation of chlorinated organic compounds.
On a pound for pound basis, C1O2 is more expensive than C12, but this is offset by the fact that
chlorine dioxide is considered to be 2.63 times more effective than C12 on a weight basis. Consequently,
less chlorine dioxide is needed to obtain the same bleaching effect.4 Recent analyses estimate that chlorine
dioxide can cost approximately two to four times as much as elemental chlorine for equivalent oxidizing
power (O'Reardon, 1992).
4 The actual equivalent bleaching power of chlorine dioxide over chlorine varies from about 2.5:1 up
to 4:1.
Page 5-7
-------
Pollution Prevention m Pulp & Paper __ Section Five Bleaching
Generation of Chlorine Dioxide
Once generated, chlorine dioxide is unstable and hence cannot be shipped. In addition, chlorine
dioxide can only be stored for a limited period of time (6 to 24 hours). Because of these constraints, C1O2
is always generated onsite at the mill. The principal feed material for C1O2 generation is sodium chlorate
(NaC103), which is produced using electrolysis technology, similar to that used in chlor-alkali production.5
The electrolysis of a sodium chloride solution generates chlorine gas, which is reacted with water to form
hypochlorite. The hypochlorite is then oxidized to form sodium chlorate, which is crystallized and dried
for shipping (SRI, 1989).
At the mill, chlorine dioxide gas is generated using various systems based on the reduction of
sodium chlorate in the presence of a reducing agent. Table 5-3 summarizes most of the chlorine dioxide
generating processes operated at North American pulp mills. The systems use a variety of reducing
agents: sulfur dioxide (Mathieson process), methanol (Solvay, R8, and modern SVP processes), and
sodium chloride (R2, R3, and SVP processes).6 A major distinction between systems is the amount and
type of byproducts produced, which can include C12, NajSC^, H2SO4, and NaCl.
Kraft pulp mills have traditionally been able to use of most of the C1O2 byproducts for bleaching
or as makeup chemicals in the recovery cycle (McKetta, 1979). With the increasing rate of chlorine
dioxide substitution, however, the generation of byproducts frequently exceeds the mills' makeup
requirements. Technologies that limit byproduct generation, therefore, have gained importance. A newer
generating technology (R8/SVP-Lite™) virtually eliminates the generation of byproduct chlorine gas by
using methanol as a reduction agent. This process was developed in the late 1980s and is now used in
many mills (Reeve, 1993).7 Some additional technologies have developed that recover or transform
byproducts that cannot be used in the recovery cycle. For example, the R9™ process, which is linked to
the R8 reactor, generates chlorine dioxide and sodium hydroxide with no byproducts. The
5 The chlor-alkali process applies electricity to a solution of sodium chloride and produces elemental
chlorine (CL,) and caustic (NaOH) in a fixed ratio.
6 The R-series generators are produced by Sterling Chemical (formerly ERGO and Albright & Wilson,
subsidiaries of Tenneco) and the SVP systems are made by Eka Nobel, Inc.
7 The R8 reactor also provides more generating capacity than the R3 using similar equipment. This
factor has contributed to the high degree of upgrading occurring in the industry.
Paee 5-8
-------
Section Five Bleaching
Pollution Prevention in Pulp & Paper
TABLE 5-3
Summary of Chlorine Dioxide Generation Processes
Process
Mathieson
Solvay
R2
R3/SVP
R6/Lurgi/Chemetics/Vulcan
R8/SVP-MeOH/SVP-Lite
Reaction Equation
2NaClO3 + S02 + H^SO, ->
2C102 + 2NaHSO,
2NaClO3 + CH3OH + H^SO, ->
2C1O2 + 21^0 + HCHO + NajSO,
NaClO3 + NaCl + 11,80, ->
C1O2 + 0.5C12 + NajSO, + R,O
NaClO3 + NaCl + t^SO,, -»
C1O2 + 0.5C12 + Na^O, + H.O
l)NaCl + 3E,O -^ NaClO3 + SHj
2)C12 + H, -> 2HC1
3)NaClO3 + 2HC1 -»
C1O2 + 0.5C12 NaCl + K,O
9NaClO3 + 2CH3OH + eHjSO,, ->
9C1O2 + 3Na3H(SO4)2 + 0.5CO2 +
1.5HCOOH + 71^0
Reducing Agent
SO2
CH3OH
NaCl
NaCl
HC1
CH3OH
Byproducts
spent acid solution
spent acid solution
spent acid and
chlorine
saltcake and chlorine
none
acid saltcake
Source: Stockburger, 1992.
Page 5-9
-------
Pollution Prevention in Pulp & Paper . Section Five - Bleaching
R6/Chemetics/Lurgi-style generators are integrated plants which generate sodium chlorate onsite from
chlorine or sodium chloride, and create no byproducts. The quality of the chlorine dioxide generated from
these integrated plants, however, is not as pure. Finally, Eka Nobel has recently introduced a process that
uses hydrogen peroxide as a reducing agent instead of methanol, which provides further increases in
capacity from the same equipment.
Equipment specifications (i.e., metallurgy) for C1O2 generators are stringent due to the extremely
corrosive and unstable nature of the product. Storage and handling of C1O2 solution must be performed
with care due to the explosion potential. In particular, contamination of feed equipment with oxidizable
materials such as rubber, grease, iron, etc. must be avoided.
The corrosiveness of chlorine dioxide also prevents the mill from recovering chemicals and
recycling process water from the D-stages. Many pulp mills are now striving to close the process water
loop in the bleaching plant. A closed water system lowers mills' costs by increasing the recovery of
makeup chemicals, reducing costs for obtaining and pumping raw water, and limiting wastewater treatment
requirements. The use of chlorine dioxide can promote chloride corrosion in mill equipment, although
to a lesser degree than elemental chlorine, thus making closed systems more difficult to achieve. In order
to increase water recycling and chemical recovery, the waste streams containing chlorinated compounds
(i.e., the C-, E-, and D-stages) should be isolated and excluded from the rest of the closed system.
Chlorine dioxide substitution in the first bleach stage may be implemented in several ways,
including: (1) addition prior to chlorine (denoted DC), (2) addition after chlorine (CD), or (3) in a mixture
with chlorine (C+D). Of these, the most effective method, in terms of bleaching efficiency, is the addition
of chlorine dioxide prior to chlorine (Teder and Tormund, 1990; Pryke, 1989). Unfortunately, this order
of addition has been linked with higher rates of dioxin and furan formation (Berry et al., 1989).
5.2.1 Number of Installations
Chlorine dioxide use has increased dramatically in recent years, as it has been adopted as the first
proven method for reducing the levels of dioxin in bleaching effluent. It is relatively easy to illustrate the
increasing use of chlorine dioxide in bleaching, since over 90 percent of U.S. demand for sodium chlorate
(the feedstock for chlorine dioxide production) is accounted for by the pulp and paper industry (SRI,
Page 5-10
-------
Section Five - Bleaching Pollution Prevention in Pulp & Paper
1991). Figure 5-1 shows the trend for apparent domestic consumption of sodium chlorate in the U.S.
between 1955 and 1987.8 By 1987, annual consumption had risen to 499,000 tons. Recent estimates by
API pegged the 1990 level of consumption at 670,000 tons per year and projected growth to 975,000 tons
per year by 1995 (API, 1992). Other sources forecast growth of roughly 8 to 10 percent per year through
1995 (Bradley, 1991).
Table 5-4 shows the degree of chlorine dioxide substitution at U.S. mills in 1988, the most recent
year for which data is available. As of mid-1988, chlorine dioxide substitution of 20 percent or greater
was being practiced on only 16 of 165 bleaching lines. Since that time, however, it is apparent that the
adoption of chlorine dioxide substitution has increased dramatically. Recent data suggests that Canadian
bleach mills have extensively adopted chlorine dioxide substitution above 70 percent (Luthe et al., 1992
cited in McCubbin, 1993). A 1992 survey of Canadian pulp mills (Pryke et al., 1992) found that 87
percent of all Canadian bleached kraft pulp was produced using significant (25 to 75 percent) or 100
percent chlorine dioxide substitution. It is generally agreed that while U.S. mills have increased their use
of chlorine dioxide, the percentage practicing "high" substitution is not as great as in Canada.
Market researchers Law, Sigurdson & Associates report that most bleached kraft mills now have
chlorine dioxide generators and are currently using them to practice 45 to 50 percent substitution. This
percentage is likely to increase to 60 to 65 percent in the future, depending upon the stringency of future
environmental regulations and the direction taken by the industry to meet them (Shapiro, 1992).
While most mills currently have chlorine dioxide generating capacity onsite, further increases in
production to enable higher substitution rates (and improved effluents) will require additional generating
capacity, which will be expensive. Many mills are now at the point where they would have to make a
significant capital investment to boost substitution any further. In fact, some companies, such as
Weyerhaeuser, have declared that they will not invest additional money in chlorine dioxide generation.
Further substitution at such mills will come through investment in non-chlorine technologies such as
extended delignification, oxygen delignification, and peroxide stages. By reducing the overall bleaching
chemical demands, these technologies will allow higher effective substitution rates using their existing
C1O2 capacity (McCubbin, 1992).
8 Apparent domestic consumption is calculated as production plus imports minus exports plus or minus
year end-stock changes.
Page 5-11
-------
tro
o
t-^i
K>
Figure 5-1
North American Consumption of Sodium Chlorate for Chemical Pulp Bleaching
1200
1000
s 800
| 600
400
200
0
SRI estimates and projections
_L
API projections
1955 1960 1965
Source: SRI (1989 and 1991), API (1992).
1970
1975
1980
1985
1990
1995
-------
Section Five Bleaching
Pollution Prevention in Pulp & Paper
TABLE 5-4
Levels of Chlorine Dioxide Substitution at
U.S. Kraft Mills, 1988
Percent
Substitution
over 50%
40 to 50%
30 to 40%
20 to 30%
10 to 20%
5 to 10%
less than 5%
0%
TOTAL
Number of Kraft
Bleach Lines
3
3
1
9
33
41
16
59
165
Percent of
Total
1.8%
1.8%
0.6%
5.4%
20.0%
24.8%
9.7%
35.7%
100%
Cumulative
Number
3
6
7
16
49
90
106
165
-
Cumulative
Percent
1.8%
3.6%
4.2%
9.7%
29.7%
54.5%
64.2%
100.0%
-
Note: Since 1988 chlorine dioxide use in the industry has expanded considerably.
Consumption for chemical pulp bleaching in 1988 was 483,000 short tons; in 1992
API projected that by 1993 consumption would rise to 920,000 tons.
(See Figure 5-1)
Source: U.S. EPA (1990a).
Page 5-13
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„ . „ . • D, ;,, s vn-nar Section Five Bleaching
Pollution Prevention in Pulp & raper _ ^ °
According to a recent report, there are approximately 166 chlorine dioxide generators in North
America, representing installed capacity of 3,194 short tons per day (Britt, 1992, cited in Stockburger,
1992). Table 5-5 summarizes the number and type of generators installed. The modern methanol-based
generating systems (R8/SVP-MeOH/SVP-Lite) account for 52 percent of the generators and 76 percent
of installed capacity.
5.2.2 Costs and Economics
The capital cost of a 30 ton per day C1O2 generating system has been estimated at $2.8 million
with an installed cost of $5.7 million (Parkinson, n.d. cited in EPA, 1990). Engineering estimates prepared
for the province of Ontario indicated a cost of $100,000 to $200,000 per daily tonne requirement or an
average of $3 to $6 million (Ontario Ministry of the Environment, 1988). Still another estimate has placed
the cost of equipment to convert sodium chlorate to chlorine dioxide at $10 to $20 million (Shariff, 1991).
Since most mills already operate their chlorine dioxide generating equipment to capacity, attempts
to increase the substitution rate by generating more C1O2 can involve investments of up to $25 million.
A Canadian study estimated the capital cost of expanding chlorine dioxide generating capacity at Ontario
mills to be $2 million to $20 million per facility (Canadian dollars) with an average cost of about $9
million (McCubbin et al., 1992). Additional investments may be required to upgrade the mixing
equipment and controls at the chlorine dioxide addition point.
Under current chlorine and chlorine dioxide prices, the cost of substituting C1O2 for C12 may
increase by as much as a factor of eight, as shown in the following calculations:
Cost of C12 approx. $45/ton or $0.0225/lb
Equivalent oxidizing power of C1O2 compared to chlorine 2.63
Cost of C12 to replace 1 Ib of C1O2 2.63 x $0.0225 = $0.06
Cost of sodium chlorate to produce 1 Ib. C1O2 $0.30
Cost of reducing agent (methanol, acid, etc.) to produce 1 Ib C1O2 $0.20
Total cost of generating 1 Ib C1O2 $0.50
Ratio of cost of 1 Ib C1O2 to equivalent C12 $0.50 -=- $0.06 = 8.3
Source: Graves, 1993.
Page 5-14
-------
Section Five - Bleaching
Pollution Prevention in Pulp & Paper
TABLE 5-5
North American Chlorine Dioxide Generators
Process
Mathieson
Solvay
R2
R3/R3H/SVP
R6/Lurgi/Chemetics
R8/SVP-MeOH/SVP-Lite
TOTAL
Number
27
17
13
14
9
86
166
Installed
Capacity (short t/d)
190
127
92
198
144
2443
3194
Percent of
N. American
Capacity
6%
4%
3%
6%
5%
76%
100%
Source: Britt, 1992; cited in Stockburger, 1992.
Page 5-15
-------
Pollution Prevention in Pulp & Paper Section Five - Bleaching
The economics of increased substitution have been explored in further detail in two recent analyses
based on engineering costing of a hypothetical mill situation:
• An analysis presented at the U.S. EPA's Symposium on Pollution Prevention in the Manufacture
of Pulp and Paper (McCubbin, 1993) compared the costs and environmental affects of alternative
upgrades to an existing 1,000 tpd model mill (see Table 5-6). One of the options considered was
an increase in chlorine dioxide substitution from 11 percent to 50 and 100 percent. At 50 percent
substitution the mill would require an incremental capital investment of $5 million, and annual
operating expenses would rise by $1.9 million. This upgrade was assumed to be achieved by
expanding the capacity of the existing chlorine dioxide generator (approximately doubling
capacity). At this point the generator would be at its maximum capacity; further expansions
would require a new generator. An upgrade to 100 percent substitution would require a capital
investment of $15.9 million (new generator among other things) and an increase in annual
operating costs of $7.1 million.
• A similar study by Brenner & Pulliam (1992) compared alternative bleaching technologies for
a greenfield mill (new installation). Their 1000 tpd model mill was assumed to be practicing 30
percent substitution under the baseline scenario. Table 5-7 shows the incremental costs and
environmental improvements that would result by modifying the mill design to use 70 percent
chlorine dioxide substitution. No decline in pulp yield would result at 70 percent substitution (i.e.,
wood chip costs would not rise). The capital costs for the mill would be less than 1 percent
higher than for the baseline mill (an incremental $0.5 million for a $284 million mill). Shifting
from 30 to 70 percent substitution would increase bleaching chemical costs by 23 percent. Total
operating costs, however, would only increase by 4 percent. The total cost for this hypothetical
greenfield mill, including both capital and operating expenses, would presumably increase by 2.6
percent with the higher substitution rate.
Table 5-8 provides yet another analysis of chlorine dioxide substitution costs. This table compares
various levels of chlorine dioxide substitution with and without an oxygen delignification stage. For the
baseline case of no chlorine dioxide substitution, chemical costs are estimated at $10.83 per ton of pulp.
Assuming oxygen delignification costs of $5.30 per ton, at 70 percent chlorine dioxide substitution
bleaching costs will be $14.02 per ton, an increase of $3.19.
These cost comparisons provide some rough estimates of the incremental costs for increasing
chlorine dioxide substitution; the actual costs will be very site-dependent. All of these examples assume
that under the baseline scenario the mills are not employing any alternative pulping technologies such as
extended or oxygen delignification. The adoption of these technologies would reduce the kappa number
of the pulp entering the bleach line and, therefore, reduce the quantity of chlorine dioxide needed in the
bleaching stages.
Page 5-16
-------
TABLE 5-6
Cost and Environmental Comparison of Chlorine Dioxide Substitution
Parameter
Incremental Capital Cost
($ Million)
Incremental O&M Costs
($ Million)
AOX in Bleach Plant Effluent
(kg/ton)
Dioxin/Furan Detect?
BOD Reduction (kg/day)
Incremental
Power
Requirements
On-Site
(MW)
Off-Site
(MW)
Chlorine Dioxide Substitution Level
Baseline Model
Mill
11% C1O2
Substitution
$0.0
$0.0
5.3
Yes
0
Maximum
Substitution
W/EQP and
existing C1O2
Capacity
$2.8
($0.5)
3.4
Perhaps
0
0
(2.6)
50% C1O2
Substitution
$5.0
$1.9
1.9
Marginal
0
0
(2.7)
100% C1O2
Substitution
$15.9
$7.1
2.1
No
0
0
5.3
100% C1O2
Substitution
(W/EQP)
$13.6
$3.2
1.5
No
0
0
1.7
Note: Baseline mill is a 1000 tpd softwood kraft pulp mill. Bleach sequence is CD-E-D-E-D.
Source: McCubbin, 1992.
T3
P
era
-------
Pollution Prevention in Pulp & Paper
Section Five Bleaching
TABLE 5-7
Cost and Environmental Comparison of Chlorine Dioxide Substitution
Greenfield Mill
Parameter
Incremental Capital Cost
($ Million)
Incremental O&M Costs
($ Million)
Total Incremental Cost ($ Million)
Pulp Yield
AOX in Bleach Plant Effluent
(kg/metric ton)
BOD in Bleach Plant Effluent
(Ibs/bleached ton of pulp)
Effluent Color
(Ibs/bleached ton of pulp)
Bleaching Scenario
Baseline Model Mill
30% C1O2 Substitution
$283.88
$180.53
$285.53
92.1%
4.1
37
298
70% C1O2 Substitution
$284.39
$187.77
$292.96
92.1%
2.8
37
176
Note: Baseline mill is a greenfield 1000 tpd softwood kraft pulp mill. Baseline bleach sequence is
Source: Brunner and Pulliam, 1992.
Page 5-18
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Section Five Bleaching
Pollution Prevention in Pulp & Paper
TABLE 5-8
Impact of Chlorine Dioxide Substitution Levels
on Chemical Requirements and Costs
Is Oxygen Delig-
nification Installed
1 No
2 No
3 Yes
4 Yes
5 Yes
6 No
7 Yes
% C1O2
Subst.
0
10
50
60
70
70
100
C12
(tpd)
41.69
37.52
10.65
8.52
6.39
12.51
0.00
C102
(tpd)
0.00
1.59
4.05
4.86
5.67
11.10
8.11
Total
Chemical
($/ton)
$10.83
$11.38
$7.16
$7.44
$7.72
$14.67
$8.56
O2 delig.
($/ton))
$0.00
$0.00
$6.30
$6.30
$6.30
$0.00
$6.30
Total
($/ton)
$10.83
$11.38
$13.46
$13.74
$14.02
$14.67
$14.86
Source: Devlin (1991); cited in Bettis (1991).
Page 5-19
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Pollution Prevention in Pulp & Paper Section Five - Bleaching
The cost of adopting increased chlorine dioxide substitution will also vary according to the
condition and type of the existing generator. Pre-1970s equipment does not lend itself well to capacity
expansions. To boost substitution rates, a new generator would be required. Much of the pre-1970s
equipment, however, would probably be corroded and due for replacement. The newer design generators
installed in the early 1980s (i.e., R3/SVP type) can be upgraded relatively inexpensively. According to
McCubbin (1993), the capacity on these generators could probably be doubled for a few million dollars.
Beyond that point, however, a new generator would be required, costing in the neighborhood of $10 to
$20 million.
The capital and operation and maintenance (O&M) expenses for new chlorine dioxide systems
will also vary depending upon the type of new capacity installed. Some of the newer integrated C1O2
plants have a high capital cost, but reduced O&M expense. Power costs constitute the majority of the
operating expense for integrated plants. In contrast, non-integrated chlorine dioxide generators, which rely
on an external source of sodium chlorate, require less capital but incur higher O&M expenses
(Stockburger, 1992). Note that the energy content of purchased sodium chlorate is high also, but that
much of the energy is consumed in Canada, a major source of chlorate for U.S. mills.
The adoption of an oxygen/peroxide reinforced extraction stage (EOP) following the first bleach
stage can have a significant impact on the cost and effectiveness of chlorine dioxide substitution.
Although an EOP stage is more expensive than traditional caustic extraction, its contribution to
delignification reduces the demands for chlorine dioxide in the final bleaching stages. This in turn frees
up chlorine dioxide capacity for the first bleaching stage, hence, the substitution rate can be boosted
without building new generating capacity. EOP stages are very economical, thus most mills would adopt
E0p before making any other modifications. The cost of oxygen for extraction is cheaper than chlorine
dioxide and the capital investment would be limited to roughly $1 million. Similarly, increased
competition and capacity have recently brought peroxide prices down significantly (from $2.00 per kg
three to four years ago to $0.75 per kg today) (McCubbin, 1993).
5.2.3 Pollution Prevention Potential
A number of interrelated environmental issues are associated with higher levels of chlorine dioxide
substitution. These are discussed below.
Pase 5-20
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Section Five Bleaching Pollution Prevention in Pulp & Paper
Impact on Effluent Quality
The amount of chlorinated organics formed during bleaching is proportional to the quantity of
atomic chlorine consumed in the first bleaching stage. Both elemental chlorine and chlorine dioxide
contain atomic chlorine, of which roughly 10 percent will end up as AOX. The substitution of chlorine
dioxide for elemental chlorine is effective at reducing the formation of AOX for several reasons: (1)
chlorine dioxide contains only one-half the atomic chlorine as elemental chlorine, and (2) less chlorine
dioxide is needed, since it contains 2.63 times the oxidative power as elemental chlorine (McDonough,
1992). Overall, chlorine dioxide bleaching results in only one-fifth the amount of chlorinated organics
as traditional chlorine bleaching (Forbes, 1992).
Brenner and Pulliam's study (1992) shows that shifting from 30 percent to 70 percent substitution
would reduce chlorinated discharges and color, but would have little impact on BOD5 (see Table 5-7).
AOX in the bleach plant effluent would decline by 32 percent; color in the effluent would decline by 41
percent; but no improvement in BOD5 would be seen. The study suggests, however, that at substitution
rates above 70 percent improvements in BOD5 could result.
McCubbin's (1993) analysis, which models the impacts of upgrading a hypothetical mill from an
11 percent substitution rate, suggests similar improvements in bleach plant effluent (see Table 5-6).
Increasing substitution from 11 percent to a maximum of 30 percent (using EOP and existing generating
capacity) would reduce AOX to 3.4 kg per ton (a 36 percent decline) and possibly eliminate detectable
dioxin and furan. Moving from 11 to 50 percent substitution reduces bleach plant effluent AOX levels
from 5.3 kg per ton to 1.9 kg per ton (a 64 percent decline), reduces dioxin/furan levels from detectable
to "marginally" detectable, and has no discernible affect on BOD. Increasing substitution from 11 to 100
percent reduces AOX from 5.3 kg per ton to 2.1 (a 60 percent decline), dioxin/furans would become non-
detectable, and no improvements in BOD would be seen. Adding an oxygen/peroxide reinforced
extraction stage following the 100 percent chlorine dioxide bleaching stage would further reduce AOX
to 1.5 kg per ton (McCubbin, 1993).
Axegard (1987) investigated the relationship between the level of chlorine dioxide substitution and
the amount of chlorinated phenolics in bleaching effluent. Chlorinated phenolics first increased slightly
with the degree of substitution. Once a level of approximately 50 percent substitution was reached,
Page 5-21
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Pollution Prevention in Pulp & Paper Section Five - Bleaching
however, emissions decrease rapidly. This same study also measured AOX as a function of the degree
of C1O2 substitution but found a linear relationship. This illustrates the fact that under varying bleaching
conditions the individual chlorinated organics may be found in different ratios. Given these findings, the
choice of regulatory parameter could influence the selection of technology.
Impacts on Generation of Chloroform
Chlorine dioxide use is also linked to the formation of chloroform, a volatile organic and a toxic
air pollutant. In a study for the National Council for Air and Stream Improvement, the environmental arm
of the U.S. pulp and paper industry, Crawford et al. (1991) investigated the impacts of increasing chlorine
dioxide substitution on the formation of chloroform. The authors found that total chloroform emissions
from the bleach plant vents and the acid and alkaline sewers fell from below 0.35 kg per adt at 15 percent
substitution to below 0.01 kg per adt at 100 percent, though adding C1O2 before chlorine increased the
amounts by 1.6 to 4.9 times. Thus, high chlorine dioxide substitution can be very effective in reducing
emissions of chloroform.
Byproducts from Chlorine Dioxide Generation
Chlorine dioxide is generated by a variety of commercial processes, each of which produces some
byproducts which may themselves have potential environmental impacts. The Mathieson process, for
example, uses sulfur dioxide (SOj) as a reducing agent and generates chlorine gas as a byproduct. Sulfur
dioxide is either generated onsite by burning sulfur, or is shipped in. The R2, R3 and Hooker SVP
processes use sodium chloride, and produce chlorine gas as a byproduct.9 Hypochlorite can also be
produced as a byproduct in chlorine dioxide generators. At one time hypochlorite was used in the
bleaching process, however, it is rapidly being abandoned at most kraft mills because of its impact on
chloroform formation.
9 The byproduct chorine produced by these reactors is quite dilute, and is not of a form suitable for
pulp bleaching. Instead, the chlorine is absorbed in sodium hydroxide to convert it to hypochlorite. As
noted elsewhere i this report, however, use of hypochlorite is being phased out in the industry.
Page 5-22
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Section Five Bleaching Pollution Prevention in Pulp & Paper
At high substitution rates, the generation of byproducts such as sodium sulfate and sulfuric acid
will exceed the mill's capacity to incorporate them in the chemical recovery cycle. It is likely that excess
byproducts would be discharged to the sewers where it would be subject to whatever treatment was in
place. The impact of these discharges will vary depending on the quantities and the type of treatment
practiced, however, in most situations it would simply act to neutralize acids. This possibility was
considered in the study for Ontario, where the authors concluded "...we do not consider the discharge of
unusable chlorine dioxide generator byproducts... to be of any environmental consequence." (Ontario
Ministry of the Environment, 1988).
One further issue that has been discussed concerns the potential for formation of chlorates during
C1O2 bleaching. Axegard (1987) showed that higher levels of C1O2 substitution increased the production
of chlorates, a substance that has been found in Sweden to be toxic to certain types of algae (Rosemarin
et al., 1990).10 Greenpeace (Kroesa, 1991) reports that although the impact of chlorates in North American
waters has not been investigated, the Canadian industry (PAPRICAN) has claimed that they are not
harmful to green algae such as are found in the Great Lakes. Chlorate can be converted to harmless
chloride in a suitably operated biological treatment system.
Impacts on Energy
The adoption of higher levels of chlorine dioxide substitution has the potential to cause some
shifting of energy consumption patterns in the industry. The generation of elemental chlorine is less
energy intensive than all alternative bleaching chemicals except oxygen. Substitution of chlorine dioxide
for chlorine would result in overall increases in energy consumption, although most of this would take
place offsite. McCubbin's (1993) comparison revealed that the adoption of chlorine dioxide substitution
has no effect on the mill's onsite energy consumption; however, offsite energy consumption (embodied
in the purchased sodium chlorate) would increase by roughly 5.3 MW with 100 percent substitution (see
Table 5-6). For perspective, a typical 1000 tpd pulp mill probably consumes 50 MW of electricity
(McCubbin, 1993). The additional energy would be consumed during the offsite electrosynthesis of
sodium chlorate. Since some 48 percent of the chlorate is imported from Canada (SRI, 1989), much of
the energy consumption would be shifted from the United States to Canada.
10 At one time, sodium chlorate was used as a defoliant to remove the leaves from cotton and soybeans
prior to mechanical picking (Kirk-Othmer, 1979).
Page 5-23
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Pollution Prevention in Pulp & Paper Section Five Bleaching
5.2.4 Other Impacts
Potential Impacts on Chemical Markets
The increased substitution of chlorine dioxide for chlorine threatens to upset the traditional
production balance between chlorine and sodium hydroxide (caustic soda). Conventional pulp bleaching
has historically accounted for roughly 20 percent of the consumption of both of these products (TCI,
1990). Since the two chemicals are produced jointly by the chlor-alkali process in fixed ratios, declines
in chlorine demand not accompanied by a corresponding decrease in caustic requirements could create
chlorine surpluses or caustic shortages. The American Paper Institute (API) reports that demand for
caustic in pulp bleaching is expected to decline by 7 percent from 1990 to 1995, versus declines in the
demand for chlorine of roughly 35 percent (API, 1992). This forecasted imbalance within the pulp and
paper industry may be slightly offset by increases in chlorine demand from other market segments.
Several processes are either currently available or under development that could help alleviate any
potential imbalance. One technique involves producing caustic soda through the chemical conversion of
soda ash and lime, which avoids chlorine as a byproduct. Major producers such as Tenneco Minerals,
FMC Corporation, and Texas Gulf are currently manufacturing caustic soda in this manner (Busch, 1992).
In some applications, including pulp bleaching, soda ash can also be used to replace caustic soda. This
may help to buffer any caustic soda price increases precipitated by a drop in chlorine demand. Other
processes are focussing on production of caustic soda at the mill, including production of caustic as a
byproduct of chlorine dioxide generation.
On the chlorine side, attempts are being made to develop processes that can produce chlorine
dioxide from chlorine. This would tend to stabilize chlorine demand. Eka Nobel of Sweden is working
on an electrolytic process that, while promising, requires considerable onsite energy. Some of the
integrated chlorine dioxide can reportedly operate using only chlorine as a feedstock (Stockburger, 1992).
Increasing demand for chlorine derivative products such as polyvinylchloride (PVC) could also help to
maintain balance in chlor-alkali markets.
Pase 5-24
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Section Five Bleaching Pollution Prevention in Pulp & Paper
Impacts on Safety
A further issue arising from chlorine dioxide substitution is safety concerns. Chlorine dioxide is
very unstable and must be produced, stored, and used under controlled conditions. Decomposition can
occur easily if the material becomes contaminated, particularly if there is exposure to oxidative materials.
Concerns for workers have also been expressed because chlorine dioxide leaks reportedly cannot be
detected as easily as chlorine (Kroesa, 1991). However, the track record over four decades of industrial
use has been good.
5.3 SPLIT ADDITION OF CHLORINE CHARGE/IMPROVED pH CONTROL
The technique of splitting up the addition of chlorine in the C-stage follows from research by
Westvaco Corp. into ways to reduce the formation of chlorinated organics (Rise and Hintz, 1989). The
research has focused on close control of the chlorine concentration in the chlorination stage as a means
for reducing formation of chlorinated organics (in the form of AOX). This is in contrast to other
approaches that may emphasize reducing the total amount of chlorine used.
By splitting the chlorine addition into several charges, introduced at multiple points throughout
the reaction, it is believed that oxidation reactions between lignin and chlorine will be favored over
substitution reactions. Substitution reactions are associated with the formation of chlorinated organics,
while oxidation reactions are not.
The control of pH in the chlorination stage has also been used as a means for influencing the type
of pulp reactions that occur. At higher pH, more of the chlorine is converted to hypochlorous acid
(HOC1), a more powerful oxidizing agent. In the absence of other modifications, higher pH would also
reduce pulp yield. For this reason, pH control is combined with split chlorine addition to reportedly
reduce chlorinated organics formation without loss of yield.
Page 5-25
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Pollution Prevention in Pulp & Paper Section Five Bleaching
5.3.1 Number of Installations
Westvaco has implemented split chlorine addition at its bleached kraft mills at Luke (Maryland),
Covington (Virginia), and Wickliffe (Kentucky). To our knowledge, no other pulp and paper companies
have so far indicated they are experimenting with this technique.
53.2 Costs and Economics
Separate costs for conversion of the chlorination stage to implement split chlorine addition and
further pH control have not been reported. The necessary equipment for splitting the chlorine charge and
for monitoring pH are likely to be quite modest.
53 J Pollution Prevention Potential
Westvaco's studies indicate that split chlorine addition using three smaller chlorine charges reduced
the formation of 2,3,7,8-TCDD and 2,3,7,8-TCDF by 70 and 50 percent, respectively. By incorporating
advanced pH control, these discharges reportedly fell by 90 percent (see Table 5-9). Mill trials have
shown that fairly low levels of TCDD and TCDF (6 ppt in CE-stage pulp) can be obtained (Hise, 1989).
5.4 OXYGEN-REINFORCED EXTRACTION
The term oxidative extraction (or oxygen extraction) refers to the use of elemental oxygen in the
first alkaline extraction stage (E^ of a conventional bleaching sequence. In a conventional bleaching
sequence, the extraction stage follows chlorination and completes the solubilization of chlorinated and
oxidized lignin molecules, facilitating their removal. The addition of gaseous oxygen to the alkaline
extraction stage can enhance the removal of lignin and provide additional bleaching power, thereby
reducing the requirements for chlorine and chlorine dioxide. Chlorine dioxide savings of approximately
2 kg per ton of pulp in subsequent D-stages are normal. Delignification following first-stage chlorination
and extraction has been found to increase by approximately 25 percent (O'Reardon, 1992). Oxidative
extraction has also been used to help mills cut back on hypochlorite, a more expensive and aggressive
chemical and the one that is most associated with chloroform emissions.
Page 5-26
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Section Five Bleaching
Pollution Prevention in Pulp & Paper
TABLE 5-9
Effect of Split Chlorine Addition on
Formation of TCDD and TCDF
(ppt)w
Method of Addition of
Chlorine
One charge
Two charges
Three charges
2,3,7,8-TCDD
31.7
18.6
14.8
2,3,7,8-TCDF
338
187
100
(a] As measured in pulp following chlorination and first extraction stages.
Source: ffise (1989).
Page 5-27
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Pollution Prevention in Pulp & Paper Section Five - Bleaching
When oxygen is used in extraction, the bleaching stage is denoted as E0. Oxygen-enhanced
caustic solutions can also be used as pretreatments prior to chlorination, however these are not discussed
in this section. Addition of oxygen in the El or subsequent extraction stages typically occurs at a 0.5
percent on pulp basis.
A recent development has been the conversion of E-stage equipment to operate with a pressurized
pre-retention tube (Hastings et al. 1992). Savings of approximately 11 kg of active chlorine per ton pulp
were obtained and AOX fell by approximately 0.5 kg per ton (Hastings et al., 1992).
Several integrated mills producing pulps with 80 to 85 percent ISO brightness have reportedly
been able to convert to short sequence bleaching (C^oD) following installation of an E0 stage
(O'Reardon, 1992).
5.4.1 Number of Installations
Since its introduction in the late 1970s, oxygen extraction has been widely adopted in North
America and elsewhere. In 1985 it was reported that 55 mills were using E0 stages (Reeve, 1985)
representing capacity of around 15 million tpd. Since then, oxygen reinforcement has spread dramatically.
Data for Canadian mills from 1987 indicated that 80 percent of bleach lines were using oxidative
extraction (Ducey, 1987). A similar high percentage of U.S. mills are believed to have implemented
oxidative extraction. Hastings et al. (1992) estimate that in 1992 mills representing close to 60 million
tpd of the world's bleached pulp capacity had adopted E0.
5.4.2 Costs and Economics
Costs of oxygen-reinforced alkali extraction will include the costs of oxygen mixing equipment,
which can range from relatively minor to moderately expensive. According to Hastings et al. (1992), an
upflow extraction tower (or upflow pre-retention tube in front of a downflow extraction tower) is
necessary to ensure the hydrostatic pressure needed to keep oxygen hi suspension. Oxygen is added via
a high-intensity mixer or sparger at the discharge of the medium consistency pump. Between 4 and 6 kg
of oxygen per ton of pulp are normally applied. Hastings et al. (1992) also suggest that it may be
Page 5-28
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Section Five - Bleaching Pollution Prevention in Pulp & Paper
worthwhile to install an upgraded washer following this stage since it does perform a delignification role.
One source reported estimated costs for a 1983 installation at a 450 tpd mill at $0.5 million
(Ducey, 1984). These costs were reportedly more than offset by the savings in hypochlorite alone. Pryke
(1985) reported on research by PAPRICAN that showed savings of 6 kg of hypochlorite and 4 kg of
chlorine dioxide per ton of pulp in a CDEHDED sequence. Annual cost savings per mill of $600,000 to
$800,000 were also reported, but these cost data are probably outdated. They serve to demonstrate,
however, that the addition of oxygen to a traditional E-stage is normally economically attractive.
Conversion to a pressurized extraction E0 stage (Hastings et al., 1992) required installation of a
pressurized pre-retention tube, a medium consistency pump, and an oxygen sparger (see Figure 5-2). Total
costs of that project, including engineering and installation, were under $2 million.
5.43 Pollution Prevention Potential
As explained in earlier sections, the formation of chlorinated organic compounds is primarily
associated with the use of elemental chlorine in the first bleaching stage. Since the Ej stage follows the
application of elemental chlorine, the benefit accrues if the extraction stage performs a delignification role
and enables a reduction in chlorine use or an increase in chlorine dioxide substitution. Savings in active
chlorine of about 2 kg per kg of oxygen charged will generally be observed (Hastings et al., 1992). In
an atmospheric tower up to 4 kg of oxygen per tonne could be charged, while in a pressurized reactor up
to 6 kg per tonne could be added. This results in a reduction in formation of chlorinated organics.
Oxygen extraction has proven effective in reducing loadings of other pollutants. In particular, E0
has been widely adopted as a means for reducing hypochlorite consumption and, consequently, chloroform
formation (Ducey, 1984). Hypochlorite bleaching stages are most closely associated with emissions of
chloroform (Dallons et al., 1990). To the extent that oxidative extraction is used to replace hypochlorite,
improvements in atmospheric emissions of chloroform should result.
Page 5-29
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Pollution Prevention in Pulp & Paper Section Five - Bleaching
5.5 PEROXIDE EXTRACTION
Peroxide (HjOj) is often combined with caustic in the El extraction stage. In such bleaching
sequences, the symbol EP is used. As with oxidative extraction (E0), the use of hydrogen peroxide has
been found to promote additional removal of lignin during the extraction stage. The oxidative power of
the peroxide can also perform some bleaching. The combination of these effects may enable the mill to
reduce usage of chemicals either upstream, in the case of chlorine, or downstream, in the case of
hypochlorite or chlorine dioxide. Peroxide and oxygen are often used together in the E^ stage, i.e., EOP.
Hydrogen peroxide is also being used in the high density storage chest as a brightening agent. In some
cases, this is considered a safeguard against possible heat-related brightness reversion that may occur with
higher rates of chlorine dioxide substitution, particularly for high brightness grades of pulp (Downs, 1990).
Hydrogen peroxide is produced via a process called anthraquinone autoxidation, which involves
the reduction (hydrogenation) of alkyl anthraquinones to anthrahydroquinones. This product is then
oxidized to yield hydrogen peroxide and the original alkyl anthraquinone. The hydrogen peroxide is
extracted from the reaction solution with water, diluted to proper concentrations, and marketed as an
aqueous solution.
Peroxide is produced offsite and shipped to the mill via tanker as a 70 percent solution in water.
Peroxide is generally added at the inlet of the oxygen mixer when oxygen is used, and at the inlet of the
stock pump when there is no oxygen. Since it decomposes to water and oxygen gas, peroxide is
essentially environmentally benign.
Webster (1990) described a strategy to achieve significant environmental and economic benefit
from the use of peroxide, without increasing C1O2 requirements. Using a CpEoDED mill as an example,
the first step is to add peroxide to the last extraction stage. This permits reductions in chlorine dioxide
use in the final D-stage to be made without sacrificing pulp brightness. Then, peroxide is added to the
first E-stage, enabling the mill to cut the chlorine charge to maintain the same chlorination-extraction
kappa number. Finally, the chlorine dioxide saved in the final D-stage can be shifted forward to raise the
chlorine dioxide substitution in the CD-stage to a higher level. This will accomplish the goal of lowering
elemental chlorine without the need for expensive increases in chlorine dioxide capacity and without
sacrificing pulp quality.
Page 5-30
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Section Five - Bleaching
Pollution Prevention in Pulp & Paper
Vacuum
Washers
From
Proceeding
Stage
To
Next Stage
Conventional alkaline extraction stage
From
First Stage
Steam
Pre-Retention
Tube
15mln
70°C
0.4 MPa
Hot
Water
Vacuum
Washer
To
Next Stage
NaOH
MC Pump MC Mixer
Oxygen-reinforced alkaline extraction stage
Figure 5-2. Modification of extraction stage for oxygen reinforcement.
Alkaline
Sewer
Source: Hastings et al., 1992.
Page 5-31
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Pollution Prevention in Pulp & Paper Section Five - Bleaching
Recent mill trials using peroxide in the oxygen-reinforced E[ stage and in subsequent extraction
stages showed that at constant CE kappa number and chlorine dioxide consumption the use of elemental
chlorine could be reduced by 30-40 percent (Anderson, 1992). Results from trials at five mills are shown
in Table 5-10. In all of these trials, dioxin levels were reported as non-detectable. Peroxide extraction
can also assist mills in moving to 100 percent C1O2 substitution. Many mills encounter higher bleaching
costs at 100 percent substitution due to the reduced delignification efficiency compared to lower C1O2
substitution levels. Brightness ceilings may fall a little if oxygen delignification is used. Peroxide
extraction can offset these disadvantages by reducing C1O2 generating requirements and improving final
brightness (Anderson, 1992).
Although more commonly used for brightening mechanical pulps, peroxide has been used as a full
bleach stage in low-chlorine and chlorine-free bleaching sequences. Peroxide would be an essential part
of any totally chlorine-free, full brightness sequence using ozone, e.g., OZEP. Full peroxide stages
typically require 2.5 percent peroxide or more on pulp, 70°C or higher and a retention time of 2 to 4
hours. The brightening effect is increased as the time and temperature are increased but viscosity begins
to drop. To protect the pulp, a prior chelation stage is crucial to ensure removal of metal ions that remain
with the pulp as chlorine is removed from the sequence. The only kraft mill currently producing "full"
brightness chlorine-free pulp is at Monsteras, Sweden. The pulp produced is 88 percent brightness and
is made from hardwood. The mill has not yet produced full market softwood pulps, but has announced
plans to do so.
5.5.1 Number of Installations
The percentage of all bleach lines in North America using hydrogen peroxide in the extraction
stage was estimated several years ago to be 25 percent (Ducey, 1987). Strunk (1990) estimates that 60
U.S. and Canadian mills use approximately 28,000 tpy of peroxide for enhanced extraction. At an average
application rate of 0.25 percent by weight on pulp, he estimates that 11 million tons of pulp are bleached
using peroxide. Walsh (1991b) reports a 30 percent increase in peroxide use in pulp and paper between
1989 and 1990, however its use is split between bleaching of kraft, mechanical, and secondary fibers, with
about half of the U.S. consumption accounted for by kraft bleaching (Downs, 1990).
Page 5-32
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Section Five Bleaching
Pollution Prevention in Pulp & Paper
TABLE 5-10
Impact of Use of Peroxide in Extraction Stages
on Chlorine Consumption and Substitution Rate
Parameter
Before
Sequence
BS Kappa
C1O2 Subst. (%)
Kappa factor
Brightness
After
Sequence
BS Kappa
C1O2 Subst. (%)
Kappa factor
Brightness
H2O2 addition (%)
C12 decrease (%)
Mill A
CoEoDED
32
6.9
0.21
91
C-pEopDEpD
32
27
0.16
91
0.5
37
MU1B
CpEoHDED
33
8
0.16
90
CDEop(H)DEp
D
33
18
0.13
90
0.45
29
MillC
CoEoDED
28
6
0.23
91
CjjEopWipD
28
12
0.17
91
0.5
31
MillD
CuEoHDED
29
30
0.21
90
CnEopDEpD
29
47
0.18
90
0.25
33
MillE
CoEoDED
33
30
0.22
90
CcEopDED
33
47
0.17
90
035
41
Source: Anderson (1992).
Page 5-33
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Pollution Prevention in Pulp & Paper Section Five Bleaching
5.5.2 Costs and Economics
Capital costs of $100,000 for storage tanks, mix tank, and piping have been cited as typical by
Webster (1990). He notes that, compared to other options such as increased chlorine dioxide generating
capacity, the costs are much lower. In terms of operating costs, the main impacts will be a change in
overall chemical consumption. Peroxide application rates between 0.1 and 1.0 percent by weight on pulp
(2 to 20 Ibs per ton) have enabled mills to reduce their consumption of other bleaching chemicals. In
most cases, however, the overall change in chemical costs has not been reported.
The price of hydrogen peroxide has dropped significantly in recent years. When peroxide was
first introduced to the pulp and paper industry, the limited number of suppliers provided significant
technical support and accordingly high prices for their product. Now that peroxide capacity has expanded
and the use of peroxide has become more prevalent, the peroxide industry is far more cost competitive.
Prices for peroxide have recently dropped from roughly $2.00 per kg three or four years ago to $0.75 per
kg today (McCubbin, 1992). This drop in prices has meant that for most mills peroxide is less expensive
than C1O2, to the extent that replacement is feasible.
Althouse (1988) reported that addition of peroxide to an oxidative extraction stage (i.e., EOP) had
enabled one mill to shorten the bleaching sequence from five to three stages and realize the following
savings:
Electrical costs .... $1 to $3 per ton of pulp or $175,000 to $525,000 per year;
Steam costs $3 to $5 per ton of pulp or $525,000 to $875,000 per year;
Maintenance costs . . . $2 to $5 per ton of pulp or $375,000 to $875,000 per year.
The same source indicates that replacement of hypochlorite with peroxide can produce higher
strength pulp, thereby reducing the need for chemical strength additives.
Page 5-34
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Section Five Bleaching Pollution Prevention in Pulp & Paper
5.53 Pollution Prevention Potential
The use of hydrogen peroxide in the caustic extraction stage has been shown to increase
delignification and decrease the kappa number of pulp following chlorination and extraction. When
measured at this point in the bleaching sequence, the term CEK number (chlorination-extraction kappa
number) is used. In two cases reported by Walsh (1991a), CEK fell from 3.1 to 2.4 and from 4.1 to 3.0.
This higher degree of delignification means that bleaching chemicals can be reduced either upstream or
downstream of the extraction stage without impacting pulp brightness or other properties. Reductions in
bleaching chemical use, particularly in the first chlorination stage, are associated directly with reduced
levels of chlorinated organics.
In case 2 above, AOX and chloroform concentrations were reported before and after addition of
peroxide to the extraction stage. In the CEHD sequence, AOX levels were 5.1 kg per ADMT and
chloroform was measured at 0.352 kg per ADMT. In the modified CEPHD sequence, AOX was reduced
to 3.7 kg per ADMT (27 percent decrease) and chloroform to 0.178 kg per ADMT (49 percent decrease).
Webster (1990) has summarized recent mill trials using peroxide at various locations in the
bleaching sequence. Depending on the current bleaching sequence, the use of peroxide in the extraction
stage enabled each mill to cut back on chlorine or chlorine dioxide to effect reductions in all of these
pollutants. The most significant reductions -- over 80 percent for all five mills - were for dioxin. AOX
declined by close to 30 percent, while chloroform was reduced at two of the mills by approximately 50
percent.
Strunk (1990) presents data that shows the effect of peroxide addition to an oxygen reinforced
extraction stage (E0). The addition of oxygen to a conventional caustic extraction stage first lowered CEK
number from 3.8 to 2.9. Peroxide addition at 0.4 percent reduced this further to 2.2, while peroxide at
0.8 percent reduced it to 2.0.
Hastings et al. (1992) report that active chlorine savings of about 2 to 3 kg of chlorine per kg of
peroxide charged can be achieved through addition of 1-4 kg per adt peroxide in the E, stage, and that
a further 3 to 5 kg chlorine per kg peroxide charged could be saved through addition of up to 2 kg per
adt in the E2 stage.
Page 5-35
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Pollution Prevention in Pulp & Paper Section Five Bleaching
Impacts on Chloroform Generation
Hydrogen peroxide has been used successfully to partially or completely replace hypochlorite in
the bleaching sequence. For mills that use hypochlorite, this should result in reduced atmospheric
chloroform emissions. The effect on chloroform was observed in one case to have been reduced by 49
percent (Walsh 1991a).n
Walsh (199la) reported the impacts of peroxide extraction at three case study mills in terms of
the bleaching chemical reductions obtained, including reductions in hypo:
Mill 1: A CoEnD bleaching sequence at a southern hardwood kraft mill was converted to CD-EP-D
(e.g., replacement of hypochlorite with peroxide in the extraction stage). The substitution ratio
was 1:3.5 peroxide for hypochlorite. Further, chlorine dioxide consumption in the D stage was
reduced by 7.2 Ib per ton.
Mill 2: A CEHD bleaching sequence at a northern blended softwood kraft mill was converted to
CEpHD (e.g., peroxide addition to extraction stage). Hypochlorite consumption fell by 1.4 pound
for each pound of peroxide added, and chlorine dioxide requirements fell by 3.2 Ib per ton.
Mill 3: A CoEHD bleaching sequence at a southern pine kraft mill was converted to CpEpHD
(e.g., peroxide addition to extraction stage). Hypochlorite consumption dropped by 50 percent.
The cost implications of these chemical substitutions were not reported.
Strunk (1990) reported the effects on pulp properties of peroxide and oxygen addition to the first
extraction stage. Third stage pulp brightness (GE method) first rose from 72 to 79 when oxygen was
added to the conventional caustic extraction stage. Peroxide addition at 0.4 percent raised brightness to
82, while peroxide at 0.8 percent achieved an 83 brightness.
5.6 ADDITIONAL TECHNOLOGY OPTIONS IN THE BLEACHING AREA
In addition to the major pollution prevention technologies discussed above, several further options
should be mentioned. These may sometimes be adopted as part of the upgrades discussed above (as in
11 In the case of dissolving pulp mills, substitution of peroxide for hypochlorite has not yet been fully
demonstrated.
Page 5-36
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Section Five Bleaching Pollution Prevention in Pulp & Paper
the case of improved chemical controls or mixing equipment), while in other cases these qualify more as
water conservation measures or pollution control techniques than pollution prevention.
5.6.1 Improved Chemical Controls
The control of bleaching chemical application rates is vital to minimizing formation of chlorinated
organics such as dioxins and furans. Excess chlorination will not only increase bleaching costs but will
also lead to increased formation of these undesired byproducts. Upgraded instrumentation to monitor and
control chemical application rates will help minimize chlorination rates to within acceptable ranges. Costs
for improved chemical controls could range from $150,000 to $500,000, depending on the type of system
installed.
5.6.2 Improved Chemical Mixing
The equipment used to mix chemicals with pulp is an important factor in controlling formation
of chlorinated organics. The development of the high-shear mixer has resulted in greater consistency of
chemical mixing, resulting in reduced potential for localized excess concentrations of chlorine-based
chemicals. Pockets of excess chlorine or chlorine dioxide within the reactor vessel can lead to formation
of additional dioxins and furans. Costs for improved mixing equipment could range from $200,000 to
$500,000, depending on the size and type.
5.6.3 Jump-Stage, Counter-Current Washing
Reductions in effluent flows can be obtained in some cases by reusing the acid filtrates (from
hypochlorite or chlorine dioxide stages) as dilution and wash water for the first bleaching stage. The
technique also encompasses reuse of second extractions stage filtrates as dilution and wash water in the
first extraction stage. The major impact of this technique, where applicable, will be on reduced effluent
flows and water consumption. Some energy and steam savings will also result. These methods are
included on most new mill installations or rebuilds, while costs for retrofits will be very site-specific.
Page 5-37
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Pollution Prevention in Pulp & Paper Section Five - Bleaching
SECTION FIVE REFERENCES
Althouse, 1988. E.B. Althouse. "Hydrogen Peroxide Addition to EO Stages is Beneficial," Pulp &
Paper, June 1988.
Anderson, 1992. Ross Anderson. 'Peroxide Delignification and Bleaching," Proceedings, Nonchlorine
Bleaching Conference, Hilton Head SC, March 1992. Available from Miller-Freeman
Publications, San Francisco.
API, 1992. American Paper Institute. Report on the Use of Pulping and Bleaching Chemicals in the U.S.
Pulp and Paper Industry, June 26, 1992. New York.
Axegard, 1987. Peter Axegard. "Chlorine Dioxide Substitution Reduces the Load of TOC1," Proceedings,
1987 TAPPI Pulping Conference, p. 105.
Berry et al., 1989. R.M. Berry, B.I. Fleming, R.H. Voss, C.E. Luthe, andP.E. Wrist, "Toward Preventing
the Formation of Dioxins During Chemical Pulp Bleaching," Pulp & Paper Canada, September,
1990, p. 48.
Bradley, 1991. Rosemary F. Bradley. "Pulp Bleaching Agents and Technologies Substituting for
Chlorine at North American Pulp Mills," Chemical Industries Newsletter, SRI International
(Menlo Park, CA). September-October 1991.
Britt, 1992. M.P. Britt. Personal communication, Vulcan Chemicals, Birmingham, AL, March 24, 1992.
Cited in Stockburger, 1992.
Brunner and Pulliam, 1992. Lee Brunner and Terry L. Pulliam. "A Comprehensive Impact Analysis of
Future Environmentally Driven Pulping and Bleaching Technologies," Proceedings, 7992 TAPPI
Pulping Conference, Boston MA, November.
Busch, 1992. Gretchen Busch. "Staying Power," Chemical Marketing Reporter, Special Report: Paper
Chemicals '92, September 28, p.SR 9.
Crawford et al., 1991. Robert J. Crawford, Victor J. Dallons, Ashok K. Jain, and Steven W. Jett.
"Chloroform Generation at Bleach Plants With High Chlorine Dioxide Substitution and/or Oxygen
Delignification," Proceedings, 7997 TAPPI Environmental Conference, p. 305.
Dallons et al., 1990. Victor J. Dallons, Dean R. Hoy, Ronald A. Messmer, Robert J. Crawford.
"Chloroform Formation and Release From Pulp Bleaching," TAPPI Journal, June 1990, p. 91.
Downs, 1990. Tim Downs. "Chemical Markets Remain Mixed as Paper Industry Slowdown Continues,"
Pulp & Paper, November 1990, p. 55.
Page 5-38
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Section Five Bleaching Pollution Prevention in Pulp & Paper
SECTION FIVE REFERENCES (cont)
Ducey, 1984. Michael J. Ducey. "Oxidative Extraction at Halsey Mill Cuts Hypochlorite Consumption,"
Pulp & Paper, Octoer 1984, p. 118.
Ducey, 1987. Michael J. Ducey. "Pulp Bleaching Concerns Focus on C1O2 Generation, Effluent
Control," Pulp & Paper, June 1987, p. 89.
EPA, 1993. U.S. Environmental Protection Agency. Pulp, Paper, and Paperboard Industry - Background
Information for Proposed Air Emission Standards. Office of Air Quality Planning and Standards,
Research Triangle Park, NC. Preliminary Draft. April 1993.
EPA, 1991. Preliminary Summary Report of Questionnaire Responses for Mills that Bleach Chemical
Pulps. Revised. (Mimeo.) CB/EAD/OST/OW, U.S. Environmental Protection Agency.
Washington, D.C. October 31, 1991.
Fleming, 1992. Bruce I. Fleming. "The Organochlorine Spectrum: Mills, Public Must Discern Toxic,
Nontoxic," Pulp & Paper, April 1992, p.59-62.
Forbes, 1992. David, R. Forbes. "Mills Prepare for Next Century with New Pulping, Bleaching
Technologies," Pulp & Paper, September 1992, p. 79-90.
Graves, 1993. Comments of David P. Graves on draft Pollution Prevention Technologies for the Bleached
Kraft Segment of the U.S. Pulp and Paper Industry, U.S. EPA/OPPT, December 1, 1992. March,
1993.
Hastings et al., 1992. "Current State of the Art of E/O, E/P, and E/OP Technologies," Proceedings,
Nonchlorine Bleaching Conference, Hilton Head, SC, March 2-5, 1992. Available from Miller
Freeman Publications, San Francisco.
Hise and Hintz, 1989. Ronnie G. Hise and Harold L. Hintz. "Effect of Brownstock Washing on
Formation of Chlorinated Dioxins and Furans During Bleaching," Proceedings, 7959 TAPPI
Pulping Conference.
Kirk-Othmer, 1979. Encyclopedia of Chemical Technology, Vol. 5, 3rd edition. John Wiley & Sons.
Kroesa, 1991. Renate Kroesa. "Chlorine Dioxide - An Overview," July 1991. (Greenpeace mimeo).
Luthe et al., 1992. C.E. Luthe, P.E. Wrist, R.M. Berry. "An Evaluation of the Effectiveness of Dioxin
Control Strategies on Organochlorine Effluent Discharges from the Canadian Bleached Chemical
Pulp Industry," Proceedings CPPA Spring Conference, May, Jasper.
Page 5-39
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Pollution Prevention in Pulp & Paper Section Five - Bleaching
SECTION FIVE REFERENCES (cont)
McCubbin, 1993. McCubbin, Neil. "The Costs and Benefits of Various Pollution Prevention
Technologies in the Bleached Kraft Pulp Industry," Proceedings, International Symposium on
Pollution Prevention in the Manufacture of Pulp and Paper, August 18-20, 1992, Washington,
DC. U.S. Environmental Protection Agency, Office of Pollution Prevention and Toxics. EPA-
744R-93-002. February 1993.
McCubbin et al., 1991. Neil McCubbin, Howard Edde, Ed Barnes, Jens Folke, Eva Bergman, Dennis
Owen. Best Available Technology for the Ontario Pulp and Paper Industry. Rep. ISBN 7729-
9261-4, Ontario Ministry of the Environment, Toronto, Ontario.
McCubbin, 1992. McCubbin, Neil. Telephone communication between Eric M. Sigler of ERG and Neil
McCubbin, November 20.
McDonough, 1992. Thomas, J. McDonough. "Chlorine to Disappear from Future Paper Industry
Bleaching Sequences," Pulp & Paper, September, p. 61-71.
McKetta, 1979. John J. McKetta (ed.). Encyclopedia of Chemical Processing and Design. Vol 7:
Catalyst Carriers to Chloralkali. (Marcel Dekker: NY).
O'Reardon, 1992. Dan. O'Reardon. "Review of Current Technology Vital to Bleach Plant Modernization
Study," Pulp & Paper, April, p.124-129.
Ontario Ministry of the Environment, 1988. Kraft Mill Effluents in Ontario. Municipal-Industrial
Strategy for Abatement. March 1988.
OTA, 1989. U.S. Congress, Office of Technology Assessment. Technologies for Reducing Dioxin in the
Manufacture of Bleached Wood Pulp, OTA-BP-O-54 (Washington, D.C.: U.S. Government
Printing Office, May 1989).
Pryke, 1989. Douglas C. Pryke. "Chlorination-Stage Mixing Practices," TAPPI Journal, June 1989, p.
143.
Pryke et al., 1992. Pryke, D.C., M Dumitru, R. Cunnington, and D.W. Reeve. "A Survey of Chlorine
Dioxide Substitution in Bleached Kraft Mills in Canada," Proceedings, Canadian Pulp and Paper
Association Technical Conference, May 14-16, 1992, Jasper Alta, Can.. Cited in Reeve, 1992.
Pryke, 1985. Douglas C. Pryke. "Fraternity Gathers at the Chateau: The 1985 International Pulp
Bleaching Conference," TAPPI Journal, August 1985, p. 145.
Reeve, 1987. Douglas W. Reeve. "The Principles of Bleaching," Proceedings, 7987 TAPPI Bleach Plant
Operations Seminar.
Page 5-40
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Section Five - Bleaching Pollution Prevention in Pulp & Paper
SECTION FIVE REFERENCES (cont)
Reeve, 1993. Douglas W. Reeve. "The Emerging Technology of Chlorine Dioxide Delignification,"
Proceedings, International Symposium on Pollution Prevention in the Manufacture of Pulp and
Paper, August 18-20, 1992, Washington, DC. U.S. Environmental Protection Agency, Office of
Pollution Prevention and Toxics. EPA-744R-93-002. February 1993.
Reeve, 1985. Douglas W. Reeve. "Worldwide E Installation Survey," TAPPIJournal, November 1985
p. 142.
Rosemarin et al., 1990. Arno Rosemarin, Karl Johan Lehtinen, Mats Notini. "Effects of Treated and
Untreated Softwood Pulp Mill Effluents on Baltic Sea Algae and Invertebrates in Model
Ecosystems," Nordic Pulp and Paper Research Journal, February 1990, p. 83.
Shapiro, 1992. Lynn. Shapiro. "A Quality Quest," Chemical Marketing Reporter, Special Report: Paper
Chemicals '92, September 28, p.SR 3-8.
Shariff, 1991. Shalina Shariff. "New Challenges in Pulp and Paper," Chemicalweek, May 8 1991 p
36.
SRI, 1989. Chemical Economics Handbook Product Review: Sodium Chlorate, SRI International, March.
SRI, 1991. Chemical Economics Handbook Product Review: Sodium Chlorate, (online update). SRI
International, October 1991.
Stockburger, 1992. Stockburger, Paul. "What You Need To Know Before Buying Your Next Chlorine
Dioxide Plant," Proceedings 7992 TAPPIEngineering Conference, Boston, MA, September 14-17.
Strunk, 1990. Willaim G. Strunk. "Kraft Bleach Plants Increase Use of Hydrogen Peroxide as Benefits
Mount," Pulp & Paper, October 1990, p. 112.
TCI, 1990. The Chlorine Institute. Economic Impact of Dioxin Regulation on the Chloralkali Industry.
Prepared by Consulting Resources Corporation, November.
Teder and Tormund, 1990. "What Happens During Sequential DC Bleaching?" Proceedings, 7990 TAPPI
Pulping Conference.
Walsh, 1991a. Patricia Walsh. "Hydrogen Peroxide: Innovations in Chemical Pulp Bleaching," TAPPI
Journal, January 1991, p. 81.
Walsh, 1991b. Patricia Walsh, development manager for pulp and paper, Interox Corp.; cited in Chemical
Marketing Reporter, September 23, 1991, p. SR4.
Webster, 1990. John Webster. "H2O2-Enhanced Bleaching Strategy Cuts TOC1 Levels in Mill Effluent,"
Pulp & Paper, April 1990, p. 141.
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